AN1897
Application note
VIPower™: low-cost universal input DVD supply
with the VIPer22A-E
Introduction
In the past few years, many consumer products have been provided to the end user, such
as DVD or VCD players. Generally, their power supply requires multiple outputs to supply a
variety of control circuits: MCU, motor, amplifier, VFD.
Offline switch mode power supply regulators from ST’s VIPer
®
family combine high voltage,
avalanche rugged vertical power MOSFET with current mode control PWM circuitry. The
result is the innovative AC-DC converter, simpler, quicker, with reduced component count
and cheap.
The VIPer family complies with the “Blue Angel” and “Energy Star” norms, with very low total
power consumption in standby mode, thanks to the burst operation. This document presents
the application on DVD player power supply with the VIPer22A-E meeting the specifications
in
Table 1.
Figure 1. VIPer22A-E evaluation board
Table 1. Output specifications
Input
Universal
line
Min. 85 V
ac
Max. 265 V
ac
Output 1
5 V+/-5%
(1)
Output 2
+12 V+/-5%
(1)
Output 3
-12 V+/-5%
(1)
Output 4
-26 V+/-5%
(1)
Output 5
3.3 V+/-5%
(1)
Output 6
5 V
stb
+/-5%
(1)
Imin.
20 mA
Imax.1.5 A
Imax.
30 mA
Imax.
30 mA
Imax.50 mA
Imax.
150 mA
Imax.100 mA
1. The accuracy of +/-5% is reached for a range of load combination only. See
Section 3.2
for cross-
regulation results.
November 2014
DocID10240 Rev 2
1/17
www.st.com
17
Contents
AN1897
Contents
1
Application description and design . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1
Schematics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.1
1.1.2
1.1.3
1.1.4
1.1.5
Start-up phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Auxiliary supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Burst mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Feedback loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Primary driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2
Transformer consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2
3
Layout recommendation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1
3.2
Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4
5
6
Transformer specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
PCB layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
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Application description and design
1
1.1
Application description and design
Schematics
The overall schematic is shown in
Figure 3.
1.1.1
Start-up phase
The VIPer22A-E has an integrated high voltage current source linked to the drain pin. At the
start-up converter, it charges the V
DD
capacitor until it reaches the start-up level (14.5 V),
and the VIPer22A-E starts switching.
1.1.2
Auxiliary supply
The VIPer22A-E has a wide operating voltage range from 8 V to 42 V, respectively minimum
and maximum values for undervoltage and overvoltage protections.
This function is very useful to achieve low standby total power consumption. The feedback
loop is connected to 5 V output by D12 to regulate 5 V output. +5 V
stb
output is blocked by
Q3, so +5 V
stb
regulation is neglected. When the standby signal is present, the Q3 V
ce
cannot provide enough voltage to maintain D12 conducted, so the 5 V output is blocked,
and the +5 V
stb
output is connected to the feedback loop. In this condition the +5 V
stb
is
regulated. Thanks to the transformer structure, all the other secondary outputs and the
auxiliary voltages are pulled down to a very low level, also pulling down the total power
consumption. These features are below-indicated.
–
–
–
In normal full load, the VDD voltage must be lower than the overvoltage protection.
In short-circuit, the VDD voltage must be lower than the shutdown voltage.
Actually, this condition leads to the well-known hiccup mode.
In no-load condition, the VDD voltage must be higher than the shutdown voltage.
1.1.3
Burst mode
The Viper22A-E integrates a current mode PWM with a power MOSFET and includes the
leading edge blanking function. The burst mode allows the VIPer22A-E to skip some
switching cycles when the energy drained by the output load goes below E = (T
b
*V
in
)2 *
f
sw
/2L
p
(T
b
= blanking time, V
in
= DC input voltage, f
sw
= switching frequency, Lp = primary
Inductance). The consequence is the reduction of the switching losses in case of low load
condition by reducing the switching frequency.
1.1.4
Feedback loop
The 5 V output voltage is regulated by a TL-431 (U3) via an optocoupler (U2) to the
feedback pin. If the output voltage is high, the TL-431 draws more current through its
cathode to the anode and the current increases in the optocoupler diode. The current in
optocoupler NPN increases accordingly and the current into the VIPer22A-E FB pin
increases. When the FB current increases, the VIPer22A-E skips some cycles to decrease
turn-on time and lower the output voltage to the proper level (see
Figure 1).
The 5 V output
voltage is regulated thanks to the TL-431 reference voltage and the R8 and R9 resistive
dividers.
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Application description and design
Figure 2. VIPer22A-E FB pin internal structure
AN1897
1.1.5
Primary driver
In a flyback power supply, the transformer is used as an energy tank during the on-time of
the MOSFET. When the MOSFET turns off, its drain voltage rises from a low value to the
input voltage while the secondary diode conducts, transferring to the secondary side the
magnetic energy stored in the transformer. Since primary and secondary windings are not
magnetically coupled, there is a serial leakage inductance that behaves like an open
inductor charged at Ipk, causing the voltage spikes on the MOSFET drain. These voltage
spikes must be clamped to keep the VIPer22A-E drain voltage below the BV
dss
(730 V)
rating. If the peak voltage is higher than this value, the device is destroyed. The RCD clamp
(see
Figure 4)
is a very simple and cheap solution, but it impacts on the efficiency and on
the power dissipation in standby condition. Besides, the clamping voltage varies according
to the load current. RCD clamp circuits may allow the drain voltage to exceed the
breakdown rating of the VIPer22A-E during the overload operation or during turn-on with
high line AC input voltage. A Zener clamp is recommended (see
Figure 5).
However this
solution gives higher power dissipation at full load, even if the clamp voltage is exactly
defined.
1.2
Transformer consideration
On the electrical specifications of a multiple output transformer (cross-regulation, leakage
inductance), the main efforts focused on the proper coupling between the windings. A lower
leakage inductance transformer allows a lower power clamp to reduce the input power. It l
leads to lower power dissipation on the primary side. Auxiliary and secondary windings are
swapped in order to decrease the coupling to the primary one. The secondary windings act
as a shielding layer to reduce the capacitive coupling. Fewer spikes are generated on the
auxiliary windings, the primary and secondary windings have better coupling.
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Application description and design
Designing transformers for low leakage inductance involves several considerations:
–
–
–
–
–
Minimizing the number of turns
Keeping ratio of winding height to width small
Increasing width of windings
Minimizing the insulation between windings
Increasing coupling between windings
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