AN2252

Application circuit description

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Quasi-resonant controller L6565 controls the ESBT by sending a PWM (Pulse Width

Modulation) signal through the gate electrode. The gate electrode drives the internal

lowvoltage MOSFET which switches the emitter of the high-voltage BJT to the external pin S

(source). This is the reason why the transistor is qualified as "emitter switched". The source

pin is usually connected to the application ground.

The base (pin B) of the BJT requires a current bias proportional to the collector current (pin

C). This proportional bias may be provided by a current transformer. The current ratio

The current transformer turns ratio must be adapted to the current gain. An example of a

current bias arrangement is given in the Section 2. More details concerning the theoretical

and practical realizations of the bias circuit and the device operation are beyond the scope

of this application note. More details are mentioned in Application Notes AN1699 and

AN1889 (see Section 4), available from the STMicroelectronics website: www.st.com.

2

Application circuit description

2.1

Operating conditions

2.2

Circuit operation

Figure 5. shows the schematic of the power supply. The power supply can be supplied from

either an AC or a DC voltage source.

For demonstration purposes in case of an AC supply, power can be supplied through the J2

connector from a single-phase mains of 230VAC. In this case, the input voltage is doubled

by a voltage doubler consisting of diodes D2 and D3, and of capacitors C1 and C2.

If the input voltage source is AC with an RMS value above 240VAC, or if it is a DC source,

the power supply must be connected through connector J1. In this case diodes D2, D3 and

diodes D1, D4 form a bridge rectifier which charges the series-connected capacitors C1 and

C2. In the schematic (Figure 5.), diodes D1, D2, D3, D4 and capacitors C1, C2 are shown

only for demonstration purposes and the converter can be supplied directly from the bus

voltage in the application. The control device is a quasi-resonant controller L6565. During

normal converter operation it is supplied from the auxiliary winding T1C of power

transformer T1 through the one-way rectifier consisting of D5, C4 and C5. Resistor R5 and

Table 1.

Input/Output specifications

Description

Value

Input AC voltage range (400 VAC input)

320 - 480VAC

Input DC voltage range (400 VAC input)

450 - 680VDC

Input AC voltage range (230 VAC input)

160 - 240VAC

Input voltage frequency range

50/60Hz

Nominal output voltage

24VDC

Maximum output current

4.3A