AN2936
Application note
10 W SMPS with HV power MOSFET and the L6565
for three-phase industrial applications
Introduction
This document introduces a solution for industrial power supplies fed by a three-phase
mains. It uses a high voltage power MOSFET with 1500 V breakdown voltage to optimize
the operation of a quasi-resonant flyback converter based on the primary controller L6565.
The STEVAL-ISA034V1 demonstration board has been designed and developed for low
power applications. The board features two isolated outputs, 12 V and 5 V, and is capable of
delivering up to 10 W.
The board can be ordered through order code STEVAL-ISA034V1.
Figure 1.
10 W board prototype
AM01332v1
March 2009
Rev 1
1/10
www.st.com
AN2936
Demonstration board description
1
Demonstration board description
The STEVAL-ISA034V1 demonstration board is based on a quasi-resonant flyback
converter topology and uses the STP3N150 1500 V power MOSFET as the primary switch.
The STP3N150 employs STMicroelectronics’ proprietary high voltage Mesh Overlay
technology. Characteristic of this technology, the switch features very low R
DS(on)
per area,
low gate charge and high switching performance. The device is available in the TO-220, TO-
247 and TO-220FH packages.
The demonstration board is designed in accordance with the specifications in
Table 1.
Table 1.
Main specifications
Parameter
Input voltage range
Input frequency range
Output 1
Output 2
Output power
Safety
EMI
Value
185 to 460 Vac
50/60 Hz
12 V @ 0.6 A
5 V @ 0.55 A
10 W
EN60950
EN55014
The input section is equipped with two connectors: CON1 for 400 Vac input voltage, and
CON2 for 230 Vac input voltage. The output voltages are available on CON3, with a shared
ground between the two outputs.
The converter is controlled by the L6565, a primary controller for quasi-resonant ZVS (zero-
voltage switching) flyback converters. The device is capable of controlling power variations
in the mains voltage by means of line voltage feed-forward. In light load conditions the
device features a special function which automatically lowers the operating frequency while
maintaining operation as close to ZVS as possible. In addition to very low startup and
quiescent currents, this feature helps to maintain low consumption from the mains in light
load conditions.
The device also includes a disable function, an on-chip filter on current sense, an error
amplifier with a precise reference voltage for primary regulation and effective two-level
overcurrent protection. The transformer reflected voltage is set to 400 V, providing enough
margin for leakage inductance voltage spikes. A small RCD clamper circuit is used to limit
excess voltage on the drain of the power MOSFET. During normal operation, the L6565 is
powered by the auxiliary winding of the transformer, via diode D7. A spike killer circuit for the
auxiliary voltage fluctuations is present, but not connected (Q2, C12, R15). The primary
current is measured using an external sensing resistor (R21) for current mode operation.
Output voltage regulation is performed using secondary feedback on the 5 V output.
The feedback network consists of a programmable voltage reference, TL431, driving an
optocoupler which ensures the required insulation between the primary and secondary
sections. The optotransistor drives the feedback pin (COMP), which controls the operation
of the device. LC filters have been added on both of the outputs to reduce the high
frequency ripple, using moderate output capacitor values.
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Demonstration board description
AN2936
The flyback transformer is a layer-type based on an E25/13/7 core and N27 ferrite,
manufactured by Tronic, and ensures safety insulation in accordance with standard
EN60950. The schematic of the board is shown in
Figure 2.
The power supply has been
built on a double-sided 35 µm PCB in FR-4, size 120 x 68 mm. The bill of material is
provided in
Table 3.
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