ST AN2872 APPLICATION NOTE
Super wide-range buck converter based on the VIPer16
1 Introduction
This document describes the STEVAL-ISA010V1 demonstration board, which is designed
as an example of a simple non-isolated auxiliary power supply for a range of input voltages
from 85 VAC to 500 VAC.
There is an ever-increasing demand for small power supplies capable of working without
voltage range limitations, even at nominal levels of 400 VAC and 415 VAC, respectively. The
real voltage levels can reach 500 VAC (415 V + 20%). The major markets for this type of
SMPS are home appliances and metering.
The new STMicroelectronics™ family of monolithic converters is well-suited for this range of
input voltages, thanks to the 800 V avalanche-rugged MOSFET integrated within the same
package with the control chip. This application note describes a low power SMPS with
a buck topology using STMicroelectronics’ VIPer16 fixed-frequency off-line converter as
a main circuit. The VIPer16 device includes an 800 V rugged power switch, a PWM
controller, programmable overcurrent, overvoltage, overload, a hysteretic thermal protection,
soft-start and safe auto-restart after any fault condition removal. Burst mode operation at
light load combined with the very low consumption of the device helps to meet standby
energy-saving regulations. The significant benefit of this new chip derives from the jitter of
the switching frequency and the possibility to supply the chip directly from the DC HV bus,
so auxiliary supply is not mandatory. The VIPer16 is suitable for flyback or buck topologies,
and thanks to an internal self-supply circuit it does not require an auxiliary supply.
AN2872
Application note
Figure 1. The STEVAL-ISA010V1 demonstration board
April 2009 Doc ID 15305 Rev 1 1/23
www.st.com
Contents AN2872
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2 Main characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3 Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 Theory of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1.1 The basic principle of the buck converter . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1.2 Practical aspects of a buck converter dedicated for mains
and 3-phase input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2 Converter schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.3 PCB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.4 Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4 Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.1 Load regulation and output voltage ripple . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2 Standby . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4.3 Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.4 MOSFET voltage stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
4.5 Short-circuit behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.6 EMC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.6.1 Surge - IEC 61000-4-5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.6.2 Burst - IEC 61000-4-4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.6.3 EMI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4.7 Thermal behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
6 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
7 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2/23 Doc ID 15305 Rev 1
AN2872 List of tables
List of tables
Table 1. List of components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Table 2. Temperature of the VIPer16 at full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Table 3. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Doc ID 15305 Rev 1 3/23
List of figures AN2872
List of figures
Figure 1. The STEVAL-ISA010V1 demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Figure 2. Buck converter schematic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 3. Standard implementation of a buck converter with a monolithic device . . . . . . . . . . . . . . . . 8
Figure 4. Schematic of the step-down converter based on the VIPer16L . . . . . . . . . . . . . . . . . . . . . . 9
Figure 5. PCB layout of the buck converter (top, bottom, bottom layout) . . . . . . . . . . . . . . . . . . . . . 11
Figure 6. 12 V output load regulation for different output loads and input voltage levels
(linear regulator U2 not assembled) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Figure 7. Output voltage ripple for different input voltages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Figure 8. Alternative schematic of the input part using balancing Zener diodes instead of
resistors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 9. Standby consumption for different input voltage levels . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 10. Efficiency of the converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Figure 11. Drain voltage (CH1) and inductor current (CH4) at different input voltages . . . . . . . . . . . . 17
Figure 12. Indication of current (CH4) and drain voltage (CH1) during short-circuit . . . . . . . . . . . . . . 18
Figure 13. EMI measurements of the demonstration board. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4/23 Doc ID 15305 Rev 1
AN2872 Main characteristics
2 Main characteristics
The main characteristics of the SMPS are listed below:
●
Input:
–V
– f: 45 - 66 Hz
●
Output:
– 12 V ± 10%
–5 V ± 4%
– 150 mA total (5 V and 12 V output) output current for full input voltage range
●
Standby: 96 mW at 230 VAC
●
Short-circuit: protected
●
PCB type and size:
–FR4
– Single side 70 µm
– 27 x 45 mm
●
Isolation: non isolated - N connected to output GND
●
EMI: In accordance with EN55022 - class B
●
EMC: Surge - IEC 61000-4-5 - 2 kV
●
EMC: Burst - IEC 61000-4-4 - 8 kV
: 85 - 500 VAC
IN
Doc ID 15305 Rev 1 5/23
Description AN2872
3 Description
3.1 Theory of operation
The detailed calculation and principles of the buck converter for mains voltage operating
range is described in Section 6: References:1. and References:2. The basic ideas and most
important behaviors of the buck converter for mains applications is described in the chapters
that follow.
3.1.1 The basic principle of the buck converter
The schematic of the buck converter is shown in Figure 2. During the ON time, the control
circuit makes the high side switch T
output capacitor C
during the ON time. Assuming the voltage across C2 is constant and the
2
input voltage is constant, the voltage drop over L
over the inductor causes a linear increase in the current through inductor L
the inductor current is proportional to the inductance of the inductor and the level of voltage
drop over the inductor.
Figure 2. Buck converter schematic
conduct. The input DC voltage is connected to L1 and
1
is constant (V1 - V2). The constant voltage
1
. The slope of
1
Control
L
L
T1
C
V
1
1
1
+
+
D
1
–
I
L
C
2
V
V
1
2
Control
L
T
T
1
C
1
D
D
1
1
+
–
+
C
I
I
L
2
ON time OFF time
I
V
L
After T
L
t
t
is switched OFF, the low side switch (diode D1) conducts. If we assume, for
1
∆I
I
L
t
V
L
t
AM00359
simplification purposes, the voltage drop over the diode is zero, then the voltage drop over
inductor L
is equal to the output voltage (voltage drop over C2). Because the voltage across
1
the inductor is different (compare ON time), the slope of inductor current is also different.
The behavior of a buck converter can be expressed by Equation 1.
Equation 1
V
∆I
V2–()t
1
-------- --------- ---------- ------
ON
L
V2t
OFF
--------- ---------==
L
6/23 Doc ID 15305 Rev 1
AN2872 Description
The regulator of the circuit measures the output voltage, compares it with the reference
voltage and modifies the duration of the ON time to keep the output voltage constant.
In cases where the inductor current is operating in the continuous mode (the current does
not cross zero at full load) the duty cycle can be obtained using Equation 2. This formula
follows from Equation 1. Another method to obtain Equation 2 is to consider the buck
converter as a low pass filter (L
, C2), connected to a rectangular signal and that the low
1
pass filter generates a mean value.
Equation 2
V
------=
V
2
1
δ
3.1.2 Practical aspects of a buck converter dedicated for mains and 3-phase
input
The application of a mains or 3-phase buck converter using a simple monolithic device
results in several special conditions. A few of the most important are described in the
following paragraphs.
The operation of a buck converter such as that of the diagram in Figure 2 requires an active
high side switch. Therefore, the monolithic device (with integrated N-channel MOSFET) is
also connected on the high side (between + of bulk capacitor and inductor). The GND of the
controller connected to the source of the MOSFET refers to the high side of the inductor
(see Figure 3). This wiring of circuit causes the feedback signal not to be directly sensed
from the output due to the shift of the GND of output voltage and controller. Basically, there
are two ways to move the information regarding the output voltage from the output to the
controller. The first way is to apply an optocoupler between the output and the converter.
The additional error amplifier and reference (typically TL431 or a simple Zener diode) must
be assembled to drive the LED of the optocoupler. This method gives high precision of the
output voltage level and low load regulation. However, it also increases the cost and space
requirements. The second principle is to use a replica of the output voltage stored in the
auxiliary capacitor during OFF time. The schematic in Figure 3 shows the principle
connection of the components. The auxiliary capacitor C
from inductor L
voltage drop over both capacitors (C
to the same voltage level as capacitor C2. It can be expected that the
1
and C3) must be equal. However, in real applications
2
the voltages are not exactly the same. This difference is caused by the difference in the
discharge current of capacitors, different capacitance and different voltage drop on diodes
D
and D2. An important effect of the variance of the voltage drop of C2 and C3 is the fact
1
that only C
is charged during ON time. Due to this behavior it is possible to see a
2
theoretically unlimited increase in the output voltage at light or no load, because the energy
delivered during the ON time is higher than the total energy required by the load. Therefore,
an additional load (resistor) or voltage limiter (Zener diode) is required on the output to
protect the output capacitor and the load against overvoltage at light load.
is charged during the OFF time
3
Doc ID 15305 Rev 1 7/23
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