Power integrations TinySwitch-III, TNY275G, TNY274P, TNY276P, TNY275P Series Manual

TNY274-280

PI-4095-082205

Wide-Range
HV DC Input

D

S

EN/UV

BP/M

+

-

+

-

DC

Output

TinySwitch-III

®

TinySwitch-III Family

Energy Efficient, Off-Line Switcher with
Enhanced Flexibility and Extended Power Range

Product Highlights

Lowest System Cost with Enhanced Flexibility

• Simple ON/OFF control, no loop compensation needed

• Selectable current limit through BP/M capacitor value

- Higher current limit extends peak power or, in open
frame applications, maximum continuous power

- Lower current limit improves efficiency in enclosed
adapters/chargers

- Allows optimum TinySwitch-III choice by swapping
devices with no other circuit redesign

• Tight I2f parameter tolerance reduces system cost

- Maximizes MOSFET and magnetics power delivery

- Minimizes max overload power, reducing cost of
transformer, primary clamp & secondary components

• ON-time extension – extends low line regulation range/
hold-up time to reduce input bulk capacitance

• Self-biased: no bias winding or bias components

• Frequency jittering reduces EMI filter costs

• Pin-out simplifies heatsinking to the PCB

• SOURCE pins are electrically quiet for low EMI

Enhanced Safety and Reliability Features

• Accurate hysteretic thermal shutdown protection with
automatic recovery eliminates need for manual reset

• Improved auto-restart delivers <3% of maximum power
in short circuit and open loop fault conditions

• Output overvoltage shutdown with optional Zener

• Line under-voltage detect threshold set using a single
optional resistor

• Very low component count enhances reliability and
enables single-sided printed circuit board layout

• High bandwidth provides fast turn on with no overshoot
and excellent transient load response

• Extended creepage between DRAIN and all other pins
improves field reliability

Figure 1. Typical Standby Application.

OUTPUT POWER TABLE

230 VAC ±15% 85-265 VAC

PRODUCT

TNY274 P or G 6 W 11 W 5 W 8.5 W
TNY275 P or G
TNY276 P or G
TNY277 P or G
TNY278 P or G 16 W 28 W 10 W 21.5 W
TNY279 P or G
TNY280 P or G

Table 1. Notes: 1. Minimum continuous power in a typical non-
ventilated enclosed adapter measured at 50
external heatsink will increase power capability
power capability in any design or minimum continuous power in an
open frame design (see Key Application Considerations). 3. Packages:
P: DIP-8C, G: SMD-8C. See Part Ordering Information.

3

Adapter

8.5 W 15 W 6 W 11.5 W
10 W 19 W 7 W 15 W
13 W 23.5 W 8 W 18 W

18 W 32 W 12 W 25 W
20 W 36.5 W 14 W 28.5 W

Peak or

1

Open

Frame

Peak or

Adapter

2

°C ambient. Use of an

1

Open

Frame

2. Minimum peak

2

EcoSmart®– Extremely Energy Efficient

• Easily meets all global energy efficiency regulations

• No-load <150 mW at 265 VAC without bias winding,
<50 mW with bias winding

• ON/OFF control provides constant efficiency down to
very light loads – ideal for mandatory CEC regulations
and 1 W PC standby requirements

Applications

• Chargers/adapters for cell/cordless phones, PDAs, digital
cameras, MP3/portable audio, shavers, etc.

February 2006

• PC Standby and other auxiliary supplies

• DVD/PVR and other low power set top decoders

• Supplies for appliances, industrial systems, metering, etc.

Description

TinySwitch-III incorporates a 700 V power MOSFET, oscillator,
high voltage switched current source, current limit (user
selectable) and thermal shutdown circuitry. The IC family uses
an ON/OFF control scheme and offers a design flexible solution
with a low system cost and extended power capability.

TNY274-280

PI-4077-013106

CLOCK

OSCILLATOR

5.85 V

4.9

V

SOURCE

(S)

SRQ

DC

MAX

BYPASS/

MULTI-FUNCTION

(BP/M)

+

-

V

I

LIMI

T

FAULT

PRESENT

CURRENT LIMIT
COMP

ARATOR

ENABLE

LEADING

EDGE

BLANKING

THERMAL

SHUTDOWN

+

-

DRAIN

(D)

REGULATOR

5.85

V

BYPASS PIN
UNDER-VOL

TAGE

1.0 V + V

T

ENABLE/

UNDER-

VOLTAGE

(EN/UV)

Q

115 µA 25 µA

LINE UNDER-VOLTAGE

RESET

AUTO­RESTART
COUNTER

JITTER

1.0 V

6.4

V

CURRENT

LIMIT STAT

E

MACHINE

PI-4078-080905

D

S

BP/

M

S

S

EN/UV

P Package (DIP-8C)

G Package (SMD-8C)

8

5

7

1

4

2

S

6

Figure 2. Functional Block Diagram.

Pin Functional Description

DRAIN (D) Pin:

This pin is the power MOSFET drain connection. It provides
internal operating current for both start-up and steady-state
operation.

BYPASS/MULTI-FUNCTION (BP/M) Pin:

This pin has multiple functions:

1. It is the connection point for an external bypass capacitor
for the internally generated 5.85 V supply.

2. It is a mode selector for the current limit value, depending
on the value of the capacitance added. Use of a 0.1
capacitor results in the standard current limit value. Use of
a 1 µF capacitor results in the current limit being reduced to
that of the next smaller device size. Use of a 10 µF capacitor
results in the current limit being increased to that of the next
larger device size for TNY275-280.

3. It provides a shutdown function. When the current into the
bypass pin exceeds 5.5 mA, the device latches off until the
BP/M voltage drops below 4.9 V, during a power down.
This can be used to provide an output overvoltage function

2

E
2/06

µF

Figure 3. Pin Configuration.

with a Zener connected from the BP/M pin to a bias winding
supply.

ENABLE/UNDER-VOLTAGE (EN/UV) Pin:

This pin has dual functions: enable input and line under-voltage
sense. During normal operation, switching of the power

TNY274-280

600

0 5

10

136 kHz

128 kHz

V

DRAIN

Time (µs)

PI-2741-041901

500

400

300

200

100

0

MOSFET is controlled by this pin. MOSFET switching is
terminated when a current greater than a threshold current is
drawn from this pin. Switching resumes when the current being
pulled from the pin drops to less than a threshold current. A
modulation of the threshold current reduces group pulsing. The
threshold current is between 60 µA and 115 µA.

The EN/UV pin also senses line under-voltage conditions through
an external resistor connected to the DC line voltage. If there is
no external resistor connected to this pin, TinySwitch-III detects
its absence and disables the line under-voltage function.

SOURCE (S) Pin:

This pin is internally connected to the output MOSFET source
for high voltage power return and control circuit common.

TinySwitch-III Functional
Description

TinySwitch-III combines a high voltage power MOSFET switch
with a power supply controller in one device. Unlike conventional
PWM (pulse width modulator) controllers, it uses a simple
ON/OFF control to regulate the output voltage.

The controller consists of an oscillator, enable circuit (sense and
logic), current limit state machine, 5.85 V regulator, BYPASS/
MULTI-FUNCTION pin under-voltage, overvoltage circuit, and
current limit selection circuitry, over- temperature protection,
current limit circuit, leading edge blanking, and a 700 V power
MOSFET. TinySwitch-III incorporates additional circuitry for
line under-voltage sense, auto-restart, adaptive switching cycle
on-time extension, and frequency jitter. Figure 2 shows the
functional block diagram with the most important features.

Oscillator

The typical oscillator frequency is internally set to an average
of 132 kHz. Two signals are generated from the oscillator: the

maximum duty cycle signal (DC

) and the clock signal that

MAX

indicates the beginning of each cycle.

The oscillator incorporates circuitry that introduces a small
amount of frequency jitter, typically 8 kHz peak-to-peak, to
minimize EMI emission. The modulation rate of the frequency
jitter is set to 1 kHz to optimize EMI reduction for both average
and quasi-peak emissions. The frequency jitter should be
measured with the oscilloscope triggered at the falling edge of
the DRAIN waveform. The waveform in Figure 4 illustrates
the frequency jitter.

Enable Input and Current Limit State Machine

The enable input circuit at the EN/UV pin consists of a low
impedance source follower output set at 1.2 V. The current
through the source follower is limited to 115

µA. When the

current out of this pin exceeds the threshold current, a low
logic level (disable) is generated at the output of the enable
circuit, until the current out of this pin is reduced to less than
the threshold current. This enable circuit output is sampled
at the beginning of each cycle on the rising edge of the clock
signal. If high, the power MOSFET is turned on for that cycle
(enabled). If low, the power MOSFET remains off (disabled).
Since the sampling is done only at the beginning of each cycle,
subsequent changes in the EN/UV pin voltage or current during
the remainder of the cycle are ignored.

The current limit state machine reduces the current limit by
discrete amounts at light loads when TinySwitch-III is likely to
switch in the audible frequency range. The lower current limit
raises the effective switching frequency above the audio range
and reduces the transformer flux density, including the associated
audible noise. The state machine monitors the sequence of
enable events to determine the load condition and adjusts the
current limit level accordingly in discrete amounts.

Under most operating conditions (except when close to no-load),
the low impedance of the source follower keeps the voltage on
the EN/UV pin from going much below 1.2 V in the disabled
state. This improves the response time of the optocoupler that
is usually connected to this pin.

Figure 4. Frequency Jitter.

5.85 V Regulator and 6.4 V Shunt Voltage Clamp

The 5.85 V regulator charges the bypass capacitor connected
to the BYPASS pin to 5.85 V by drawing a current from the
voltage on the DRAIN pin whenever the MOSFET is off. The
BYPASS/MULTI-FUNCTION pin is the internal supply voltage
node. When the MOSFET is on, the device operates from the
energy stored in the bypass capacitor. Extremely low power
consumption of the internal circuitry allows TinySwitch-III to
operate continuously from current it takes from the DRAIN
pin. A bypass capacitor value of 0.1

µF is sufficient for both

high frequency decoupling and energy storage.

E

3

2/06

TNY274-280

PI-4098-082305

0

2500 5000

Time (ms)

0

5

0

10

100

200

300

V

DRAIN

V

DC-OUTPUT

In addition, there is a 6.4 V shunt regulator clamping the
BYPASS/MULTI-FUNCTION pin at 6.4 V when current
is provided to the BYPASS/MULTI-FUNCTION pin
through an external resistor. This facilitates powering of
TinySwitch-III externally through a bias winding to decrease
the no-load consumption to well below 50 mW.

BYPASS/MULTI-FUNCTION Pin Under-Voltage

The BYPASS/MULTI-FUNCTION pin under-voltage circuitry
disables the power MOSFET when the BYPASS/MULTI­FUNCTION pin voltage drops below 4.9 V in steady state
operation. Once the BYPASS/MULTI-FUNCTION pin voltage
drops below 4.9 V in steady state operation, it must rise back
to 5.85 V to enable (turn-on) the power MOSFET.

Over Temperature Protection

The thermal shutdown circuitry senses the die temperature. The
threshold is typically set at 142 °C with 75 °C hysteresis. When
the die temperature rises above this threshold the power MOSFET
is disabled and remains disabled until the die temperature falls
by 75 °C, at which point it is re-enabled. A large hysteresis of
75 °C (typical) is provided to prevent overheating of the PC
board due to a continuous fault condition.

Current Limit

The current limit circuit senses the current in the power MOSFET.
When this current exceeds the internal threshold (I

LIMIT

), the
power MOSFET is turned off for the remainder of that cycle. The
current limit state machine reduces the current limit threshold
by discrete amounts under medium and light loads.

The leading edge blanking circuit inhibits the current limit
comparator for a short time (t

) after the power MOSFET is

LEB

turned on. This leading edge blanking time has been set so that
current spikes caused by capacitance and secondary-side rectifier
reverse recovery time will not cause premature termination of
the switching pulse.

Auto-Restart

In the event of a fault condition such as output overload, output
short circuit, or an open loop condition, TinySwitch-III enters
into auto-restart operation. An internal counter clocked by the
oscillator is reset every time the EN/UV pin is pulled low. If the
EN/UV pin is not pulled low for 64 ms, the power MOSFET
switching is normally disabled for 2.5 seconds (except in the
case of line under-voltage condition, in which case it is disabled
until the condition is removed). The auto-restart alternately
enables and disables the switching of the power MOSFET until
the fault condition is removed. Figure 5 illustrates auto-restart
circuit operation in the presence of an output short circuit.

In the event of a line under-voltage condition, the switching of
the power MOSFET is disabled beyond its normal 2.5 seconds
until the line under-voltage condition ends.

Adaptive Switching Cycle On-Time Extension

Adaptive switching cycle on-time extension keeps the cycle
on until current limit is reached, instead of prematurely
terminating after the DC

signal goes low. This feature

MAX

reduces the minimum input voltage required to maintain
regulation, extending hold-up time and minimizing the size
of bulk capacitor required. The on-time extension is disabled
during the startup of the power supply, until the power supply
output reaches regulation.

Line Under-Voltage Sense Circuit

The DC line voltage can be monitored by connecting an external
resistor from the DC line to the EN/UV pin. During power-up or
when the switching of the power MOSFET is disabled in auto­restart, the current into the EN/UV pin must exceed 25

µA to

initiate switching of the power MOSFET. During power-up, this
is accomplished by holding the BYPASS/MULTI-FUNCTION
pin to 4.9 V while the line under-voltage condition exists. The
BYPASS/MULTI-FUNCTION pin then rises from 4.9 V to

5.85 V when the line under-voltage condition goes away. When the
switching of the power MOSFET is disabled in auto-restart mode
and a line under-voltage condition exists, the auto-restart counter
is stopped. This stretches the disable time beyond its normal

2.5 seconds until the line under-voltage condition ends.

Figure 5. Auto-Restart Operation.

E

4

2/06

The line under-voltage circuit also detects when there is
no external resistor connected to the EN/UV pin (less than
~1 µA into the pin). In this case the line under-voltage function
is disabled.

TinySwitch-III Operation

TinySwitch-III devices operate in the current limit mode. When
enabled, the oscillator turns the power MOSFET on at the
beginning of each cycle. The MOSFET is turned off when the
current ramps up to the current limit or when the DC

MAX

limit is
reached. Since the highest current limit level and frequency of
a TinySwitch-III design are constant, the power delivered to the

TNY274-280

V

DRAI

N

V

EN

CLOCK

DC

DRAI

N

I

MAX

PI-2749-082305

V

DRAI

N

V

EN

CLOCK

DC

DRAI

N

I

MAX

PI-2667-082305

PI-2377-082305

V

DRAI

N

V

EN

CLOCK

DC

DRAI

N

I

MAX

load is proportional to the primary inductance of the transformer
and peak primary current squared. Hence, designing the supply
involves calculating the primary inductance of the transformer
for the maximum output power required. If the TinySwitch-III
is appropriately chosen for the power level, the current in the
calculated inductance will ramp up to current limit before the
DC

limit is reached.

MAX

Enable Function

TinySwitch-III senses the EN/UV pin to determine whether or

Figure 6. Operation at Near Maximum Loading.

not to proceed with the next switching cycle. The sequence of
cycles is used to determine the current limit. Once a cycle is
started, it always completes the cycle (even when the EN/UV
pin changes state half way through the cycle). This operation
results in a power supply in which the output voltage ripple
is determined by the output capacitor, amount of energy per
switch cycle and the delay of the feedback.

The EN/UV pin signal is generated on the secondary by
comparing the power supply output voltage with a reference
voltage. The EN/UV pin signal is high when the power supply
output voltage is less than the reference voltage.

In a typical implementation, the EN/UV pin is driven by an
optocoupler. The collector of the optocoupler transistor is
connected to the EN/UV pin and the emitter is connected to
the SOURCE pin. The optocoupler LED is connected in series
with a Zener diode across the DC output voltage to be regulated.
When the output voltage exceeds the target regulation voltage
level (optocoupler LED voltage drop plus Zener voltage), the
optocoupler LED will start to conduct, pulling the EN/UV pin
low. The Zener diode can be replaced by a TL431 reference
circuit for improved accuracy.

ON/OFF Operation with Current Limit State Machine

The internal clock of the TinySwitch-III runs all the time. At
the beginning of each clock cycle, it samples the EN/UV pin to
decide whether or not to implement a switch cycle, and based
on the sequence of samples over multiple cycles, it determines
the appropriate current limit. At high loads, the state machine
sets the current limit to its highest value. At lighter loads, the
state machine sets the current limit to reduced values.

Figure 7. Operation at Moderately Heavy Loading.

Figure 8. Operation at Medium Loading.

2/06

E

5

TNY274-280

PI-2661-082305

V

DRAI

N

V

EN

CLOCK

DC

DRAIN

I

MAX

PI-2395-030801

0

2.5 5

Time (s)

0

100

200

400

300

0

100

200

V

DC-INPUT

V

DRAIN

0

1 2

Time (ms)

0

200

400

5

0

10

0

100

200

PI-2383-030801

V

DC-INPUT

V

BYPASS

V

DRAIN

PI-2381-1030801

0

1 2

Time (ms)

0

200

400

5

0

10

0

100

200

V

DC-INPUT

V

BYPASS

V

DRAIN

PI-2348-030801

0

.5 1

Time (s)

0

100

200

300

0

100

200

400

V

DC-INPUT

V

DRAIN

Figure 9. Operation at Very Light Load.

Figure 11. Power-Up Without Optional External UV Resistor
Connected to EN/UV Pin.

At near maximum load, TinySwitch-III will conduct during
nearly all of its clock cycles (Figure 6). At slightly lower load,
it will “skip” additional cycles in order to maintain voltage
regulation at the power supply output (Figure 7). At medium
loads, cycles will be skipped and the current limit will be reduced
(Figure 8). At very light loads, the current limit will be reduced
even further (Figure 9). Only a small percentage of cycles will
occur to satisfy the power consumption of the power supply.

The response time of the ON/OFF control scheme is very fast
compared to PWM control. This provides tight regulation and
excellent transient response.

Figure 12. Normal Power-Down Timing (without UV).

Figure 10. Power-Up with Optional External UV Resistor (4 MΩ)
Connected to EN/UV Pin.

E

6

2/06

Figure 13. Slow Power-Down Timing with Optional External
(4 M

Ω) UV Resistor Connected to EN/UV Pin.

TNY274-280

Power Up/Down

The TinySwitch-III requires only a 0.1 µF capacitor on the
BYPASS/MULTI-FUNCTION pin to operate with standard
current limit. Because of its small size, the time to charge this
capacitor is kept to an absolute minimum, typically 0.6 ms. The
time to charge will vary in proportion to the BYPASS/MULTI­FUNCTION pin capacitor value when selecting different current
limits. Due to the high bandwidth of the ON/OFF feedback,
there is no overshoot at the power supply output. When an
external resistor (4 MΩ) is connected from the positive DC
input to the EN/UV pin, the power MOSFET switching will
be delayed during power-up until the DC line voltage exceeds
the threshold (100 V). Figures 10 and 11 show the power-up
timing waveform in applications with and without an external
resistor (4 MΩ) connected to the EN/UV pin.

Under startup and overload conditions, when the conduction time
is less than 400 ns, the device reduces the switching frequency
to maintain control of the peak drain current.

During power-down, when an external resistor is used, the
power MOSFET will switch for 64 ms after the output loses
regulation. The power MOSFET will then remain off without
any glitches since the under-voltage function prohibits restart
when the line voltage is low.

Figure 12 illustrates a typical power-down timing waveform.
Figure 13 illustrates a very slow power-down timing waveform
as in standby applications. The external resistor (4 M

Ω) is

connected to the EN/UV pin in this case to prevent unwanted
restarts.

No bias winding is needed to provide power to the chip
because it draws the power directly from the DRAIN pin (see

Functional Description above). This has two main benefits.
First, for a nominal application, this eliminates the cost of a
bias winding and associated components. Secondly, for battery
charger applications, the current-voltage characteristic often
allows the output voltage to fall close to zero volts while still
delivering power. TinySwitch-III accomplishes this without a
forward bias winding and its many associated components. For
applications that require very low no-load power consumption
(50 mW), a resistor from a bias winding to the BYPASS/
MULTI-FUNCTION pin can provide the power to the chip.
The minimum recommended current supplied is 1 mA. The
BYPASS/MULTI-FUNCTION pin in this case will be clamped
at 6.4 V. This method will eliminate the power draw from the
DRAIN pin, thereby reducing the no-load power consumption
and improving full-load efficiency.

Current Limit Operation

Each switching cycle is terminated when the DRAIN current
reaches the current limit of the device. Current limit operation
provides good line ripple rejection and relatively constant power
delivery independent of input voltage.

BYPASS/MULTI-FUNCTION Pin Capacitor

The BYPASS/MULTI-FUNCTION pin can use a ceramic
capacitor as small as 0.1

µF for decoupling the internal power

supply of the device. A larger capacitor size can be used to adjust
the current limit. For TNY275-280, a 1 µF BP/M pin capacitor
will select a lower current limit equal to the standard current
limit of the next smaller device and a 10 µF BP/M pin capacitor
will select a higher current limit equal to the standard current
limit of the next larger device. The higher current limit level of
the TNY280 is set to 850 mA typical. The TNY274 MOSFET
does not have the capability for increased current limit so this
feature is not available in this device.

2/06

E

7

TNY274-280

D

S

S

BP/M

EN/UV

L1

1 mH

D1

1N4007

RV1

275 VAC

F1

3.15 A

D2

1N4007

C1

6.8 µF
400 V

C6
1 µF
60 V

C2
22 µF
400 V

C10

1000 µF

25 V

C5

2.2 nF

250 VAC

C11

100

µF

25

V

+12 V, 1

A

85-265

VA

C

RTN

J4

J3

J1

J2

C7

100 nF

50 V

U1

TNY278P

C4

10 nF

1 kV

VR1

P6KE150A

NC 8

6

4

T1

2

5

1

3

D5

1N4007GP

D6

UF4003

D7

BYV28-200

U2

PC817A

VR2

1N5255B

28 V

VR3

BZX79-C11

11 V

C7 is configurable to adjust
U1 current limit, see circuit
description

*R5 and R8 are optional
components

R5*

3.6 MΩ

R3

47 Ω

1/8

W

R4

2 kΩ

1/8

W

R6
390 Ω
1/8

W

R7

20 Ω

R8*

21 kΩ

1%

R1

1 kΩ

R2

100 Ω

D3

1N4007

D4

1N4007

L2

Ferrite Bead

3.5 × 7.6 mm

PI-4244-021406

†

†

TinySwitch-III

Figure 14. TNY278P, 12 V, 1 A Universal Input Power Supply.

Applications Example

The circuit shown in Figure 14 is a low cost, high efficiency,
flyback power supply designed for 12 V, 1 A output from
universal input using the TNY278.

The supply features under-voltage lockout, primary sensed
output overvoltage latching shutdown protection, high
efficiency (>80%), and very low no-load consumption
(<50 mW at 265 VAC). Output regulation is accomplished using
a simple zener reference and optocoupler feedback.

The rectified and filtered input voltage is applied to the primary
winding of T1. The other side of the transformer primary is
driven by the integrated MOSFET in U1. Diode D5, C2, R1,
R2, and VR1 comprise the clamp circuit, limiting the leakage
inductance turn-off voltage spike on the DRAIN pin to a safe
value. The use of a combination a Zener clamp and parallel
RC optimizes both EMI and energy efficiency. Resistor R2
allows the use of a slow recovery, low cost, rectifier diode by
limiting the reverse current through D5. The selection of a
slow diode also improves efficiency and conducted EMI but
should be a glass passivated type, with a specified recovery
time of ≤2 µs.

The output voltage is regulated by the Zener diode VR3. When
the output voltage exceeds the sum of the Zener and optocoupler

E

8

2/06

LED forward drop, current will flow in the optocoupler LED.
This will cause the transistor of the optocoupler to sink current.
When this current exceeds the ENABLE pin threshold current
the next switching cycle is inhibited. When the output voltage
falls below the feedback threshold, a conduction cycle is allowed
to occur and, by adjusting the number of enabled cycles, output
regulation is maintained. As the load reduces, the number of
enabled cycles decreases, lowering the effective switching
frequency and scaling switching losses with load. This provides
almost constant efficiency down to very light loads, ideal for
meeting energy efficiency requirements.

As the TinySwitch-III devices are completely self-powered,
there is no requirement for an auxiliary or bias winding on the
transformer. However by adding a bias winding, the output
overvoltage protection feature can be configured, protecting
the load against open feedback loop faults.

When an overvoltage condition occurs, such that bias voltage
exceeds the sum of VR2 and the BYPASS/MULTIFUNCTION
(BP/M) pin voltage (28 V+5.85 V), current begins to flow into the
BP/M pin. When this current exceeds 5 mA the internal latching
shutdown circuit in TinySwitch-III is activated. This condition
is reset when the BP/M pin voltage drops below 2.6 V after
removal of the AC input. In the example shown, on opening
the loop, the OVP trips at an output of 17 V.