AN2936

Demonstration board description

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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

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.

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.

Table 1.

Main specifications

Parameter

Value

Input voltage range

185 to 460 Vac

Input frequency range

50/60 Hz

Output 1

12 V @ 0.6 A

Output 2

5 V @ 0.55 A

Output power

10 W

Safety

EN60950

EMI

EN55014