ON Semiconductor AND8098 Technical data

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AND8098/D

Low−Cost 100 mA High−Voltage Buck and Buck−Boost Using NCP1052

Prepared by: Kahou Wong

ON Semiconductor

INTRODUCTION

This application note presents low-cost high-voltage 100 mA non-isolated power supply using NCP1052 by buck and buck-boost topology. The NCP1052 is one of the latest low-cost switching controllers with integrated 700 V/ 300 mA power switch from ON Semiconductor. It is primarily designed for isolated 10 W-range flyback converter. If isolation is not needed, the IC can also be used as stepping-down buck and buck-boost converter for further cost saving by removing optocoupler and replacing the transformer by an inductor. The output current capability is 100 mA. The possible operating range is from input range between 20 Vdc and 700 Vdc to output range of 5.0 V or above with 100 mA. Typical efficiency around 65% is obtained in the 12 V buck demo board.

Advantages of the proposed circuits include:

• Comparing to flyback, buck and buck-boost eliminates

optocoupler and replaces transformer by an inductor for cost saving.

• Buck and buck-boost offers smaller voltage stress in

switches comparing to flyback. It minimizes the switching loss and increases efficiency.

• NCP105x can power up itself from the high input

voltage with wide range between 20 Vdc and 700 Vdc. It needs no extra supply circuit.

• NCP105x operates at 44, 100, or 136 kHz and

accommodates low-cost components such as aluminum electrolytic capacitors and powered-iron core magnetic.

• NCP105x offers frequency jittering for reduced

electromagnetic inference (EMI).

• NCP105x offers thermal and short circuit fault

protection.

• Simple design as no control-loop compensation is

concerned.

The proposed buck and buck-boost converters are very similar to each other. Their major difference is that buck provides a positive output voltage but buck-boost provides a negative output voltage referring to the input ground.

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APPLICATION NOTE

PRINCIPLE OF OPERATION

Figure 1 shows the proposed buck and buck-boost converters. The rectifier circuit, which consists of capacitor C3 and diode D3, is in the front end for AC or DC input voltage. Then, the NCP1052 is self-powered up from the rectified input voltage directly with a VCC capacitor C2. When the switch inside the IC is opened, there is a voltage across Drain (D) and Source (S) pins of the IC. If this voltage is greater than 20 V, an internal current source I (typ.) inside the IC charges up C2 and a voltage in C2 is built up for the operation of the IC. Comparing to the switching frequency, the V

voltage level is in a lower-frequency

7.5-8.5 V hysteresis loop. This VCC hysteresis loop is for frequency jittering features to minimize EMI and short-circuit fault timing function.

(a) Buck

2Z2

Input Output

Figure 1. Proposed Circuit Using NCP1052

2Z2

(b) Buck-boost

In Figure 2a it is noted that in the buck topology the input voltage powers up the IC through the path across the inductor L and capacitor C. This charging path passes

start

= 6.3 mA

 Semiconductor Components Industries, LLC, 2003

June, 2003 - Rev. 1

1 Publication Order Number:

AND8098/D

through the output and a low-frequency ripple will be found in the output voltage. Hence, the value of C2 is needed to be small enough to increase this charging frequency f

VCC

in order to reduce output voltage ripple because some efficiency is lost due to this low-frequency ripple.

(a) Buck

2Z2

(b) Buck-boost

start

Input Output

start

Input Output

Figure 2. Charging Current of C

In Figure 2b it is noted that in the buck-boost topology the charging current path is blocked by diode D and hence the charging of C

does not affect the output voltage directly.

However, it still affects the output voltage indirectly and slightly by adding some low-frequency noise on the inductor. Hence, small value of C

(a) Buck

(b) Buck-boost

Figure 3. Output Voltage Couples to C1 with a

Charging Current

is also wanted.

out

The function of diode D1, capacitor C1 and resistor R1 are to transfer the magnitude of output voltage to a voltage across C1 so that the IC can regulate the output voltage. In Figure 3, when the main switch inside the IC is opened and the diode D is closed. In buck, the potential of the IC reference ground (pin S) becomes almost 0 V in this moment. In buck-boost, the potential of the IC reference ground (pin S) becomes -V

in this moment. The voltage

out

in C1 will be charged to the output voltage. On the other hand, when main switch is closed and the diode D is opened, diode D and Vin+V

is reverse biased by a voltage with magnitude V

respectively. Hence, D1 does not affect the

out

normal operation of the buck and buck-boost converter.

It is noted that the instantaneous voltage in C1 can be possibly greater than the output voltage especially when output current or output ripple is too large. It directly affects the load regulation of the circuit since the IC regulates the output voltage based on the voltage in C

. In order to solve

it, larger values of L and R1 can help to slow down the charging speed o f C1. It reduces the maximum instantaneous voltage in C

so that output voltage at high output current

can be pulled up and a good regulation is made.

Larger value of L can help the load regulation but it usually unwanted because it is bulky. Hence, resistor R1 is recommended. Larger value of R

makes higher output

voltage. Hence, it is called as a “pull-up resistor” and it can help to pull up the output voltage slightly.

The voltage in C1 representing the output voltage is feedback to the feedback (FB) pin of the NCP1052 through a diode D

and zener diode Z2. When output voltage is too

high, there will be a greater-than-50 A current inserting into the feedback pin of the NCP1052. The NCP1052 will stop switching when it happens. When output voltage is not high enough, the current inserting into the feedback is smaller than 50 A. The NCP1052 enables switching and power is delivered to the output until the output voltage is too high again.

The purpose of the diode D

is to ensure the current is

inserting into the feedback pin because the switching of NCP1052 can also be stopped when there is a greater-than-50 A current sinking from the FB pin. The purpose of the zener diode Z

is to set the output voltage

threshold. The FB pin of NCP1052 with a condition of 50 A sourcing current is about 4.3 V. The volt-drop of the diode D

is loosely about 0.7 V at 50 A. Hence, the output

voltage can be loosely set as follows:

 zener  4.3 V  0.7 V

out

 zener  5V

(eq. 1)

According to (1), the possible minimum output voltage of the circuit is 5.0 V when there is no zener diode Z2.

If there is no load, the IC will automatically minimize its duty cycle to the minimum value but the output voltage is still possible to be very high because there is no passive component in the circuit try to absorb the energy. As a result,

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AND8098/D

output voltage will rise up dramatically and burn the output capacitor eventually. Hence, a zener diode Z1 or minimum “dummy” load resistor is needed to consume the minimum amount of energy as shown in Figure 1. It is also noted that when R1 pulls up the output voltage at a given output current condition, the output voltages at lower output current conditions are also pulled up. Hence, the clamping zener diode Z

is needed to be with the breakdown voltage as same

as the output voltage but it will reduce some of the ef ficiency at lower output current conditions.

DESIGN CONSIDERATION

Topology

Buck circuit is to step down a voltage. Buck-boost circuit is to step up or down a voltage. The output voltage is inverted. The maximum duty of NCP1052 is typically 77%.

Because of burst-mode control, the effective maximum duty is lower and said to be 70% roughly. When a buck converter is in continuous conduction mode (CCM), the input voltage V

and output voltage V

are related by the

out

duty ratio D.

out

 D  0.7

The relationship in buck-boost is

out



1  D



0.7

1  0.7

 2.33

Another aspect on topology is the output current. The maximum output current is always smaller than the maximum switch current in non-isolated topologies. However, in isolated topologies such as flyback the maximum output current can be increased by a transformer.

Table 1. Summary of Topology Difference Using NCP1052

Buck Buck-boost Flyback

Output voltage < 0.7 V Output current < 300 mA << 300 mA, output current is

Input voltage < 700 V

Operating mode in nominal condition

Standby ability on VCC charging current

Transformer / Auxiliary winding It is only for standby

Isolation No No Yes. Opto coupler can be

Continuous Continuous Discontinuous

Bad. The current flows through output even if there is no load

improvement or additional output

Negative & < 2.33 Vin Depending on transformer ratio

only a portion of the inductor current

 700  V  700 V

Good. The current passes through inductor only

It is only for standby improvement or additional output

out

< 10 W. It depends on operating condition and audible noise level

<< 700 V. It depends on transformer ratio

Good. The current passes through primary winding only

It is a must for the main output. Additional auxiliary winding can improve standby performance

eliminated if isolation is not needed

(eq. 2)

(eq. 3)

Burst-mode Operation

The NCP1052 is with a burst-mode control method. It means the MOSFET can be completely off for one or more switching cycles. The output voltage is regulated by the overall duration of dead time or non-dead time over a number of switching cycles. This feature offers advantages on saving energy in standby condition since it can reduce the effective duty cycle dramatically. In flyback topology, the circuit is mainly designed for discontinuous conduction mode (DCM) in which the inductor current reaches zero in every switching cycle. The DCM burst-mode waveform can be represented in Figure 4. It is similar to the pulse-width modulation (PWM) one.

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Burst mode

PWM

Figure 4. DCM Inductor Currents in Burst Mode

and PWM Control

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