AN2252

Application circuit description

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The VCC capacitors are charged during the start-up phase by the permanent current source

made up of resistors R1, R2 and R3. Thanks to the very low current consumption of L6565

during the start-up phase, the start-up current is in the order of hundreds of microamperes

which significantly reduces the power dissipated in the start-up resistors.

The complete demagnetization of the transformer core is detected by the auxiliary winding

voltage crossing zero. Resistor R6 delivers this information to the IC's internal zero crossing

detector through pin 5. Resistor R7 shifts the zero-crossing detector threshold towards a

value closer to zero for reliable zero crossing during converter start-up or under overload

conditions.

Capacitor C8 delays the power switch turn-on to the moment when the collector voltage

reaches a valley point. The primary current control circuit consists of current-sense resistors

R11, R12 and low-pass filter R9, C7 connected to the CS pin 4 of the control IC.

The primary power switch is the STC04IE170HV. It is an ESBT rated for a maximum current

of 5A and collector-to-source voltage of 1500V. The gate of the ESBT is driven directly by

the internal gate driver of U1 through pin 7. The ESBT also requires a bias current for the

base of the internal BJT. It is provided by current transformer T2 through the diode D7.

During the storage time, the collector current flows through the B-C junction for the time

required by the junction to recover from conduction. The collector current flows then through

capacitor C10 which stores the energy that will generate the initial base current spike

necessary for the next switching cycle. The value of this current spike is determined by the

voltage across capacitor C10 (which is limited by Zener diode D9), by resistor R10 and by

the resistance of the B-E junction of the internal BJT of the ESBT. Diode D6 and resistor R4

provide the bias current required to precharge C10 during the first switching cycle and

properly start the converter operation.

Since the current transformer operation may be affected by core saturation when the

voltsecond product exceeds the limit, a protection circuit consisting of R8, C9, D8 and Q2 is

inserted in the current-sense path. This circuit is a timer which watches the maximum ON

time. If the latter goes beyond a certain limit, the current-sense voltage is suddenly

increased to its maximum threshold, thus stopping the gate driver and turning off the ESBT

through the gate. Without this circuit, the current transformer core saturation would cause

the ESBT to be unsafely turned off whenever there is a lack of base current. This condition

may happen in case of an undervoltage at the input, for instance if a mains voltage drop

occurs or the power supply is unplugged. As a consequence, the ON-time would be

increased above the specified current transformer volt-second product limit.

Clamp circuit D10, D11, C3, R13 and R14 protects the ESBT switch from the voltage spikes

induced by the transformer leakage inductance.

The output voltage is controlled by an opto-isolated feedback loop consisting of U2, voltage

divider R18, R19, R20 and frequency response compensation components R17, C13 and

C14. Since most of the voltage stress was moved (by the increased flyback voltage provided

by an appropriate transformer's turns ratio) to the primary side, a 100V Schottky diode can

be used as a rectifier on the secondary side even if the nominal output voltage is 24V. This is

one of the advantages of using the quasi-resonant mode, which further helps decrease the

output rectifier loss and increases the overall converter power efficiency.

2.2.1

Bill of materials

The list of components required to build the demonstration board is shown in Table 2. Most

of the used active components are available from STMicroelectronics. Thanks to the

outstanding performance of the ESBT, the switch does not require any heat sink for this