AN2844 datasheet(8/37 Pages) STMICROELECTRONICS

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Adapter features

AN2844

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This information is used to change the set point of the pulse-by-pulse current limitation. In

the standard application circuit with L6565 as U1, an OFF-time limitation circuit with a Q3

transistor has been added. This transistor limits the maximum switching frequency of the

converter to approximately 70 kHz. The input information coming from the OUT pin is fed

into Q3 with a delay. This feature provides some anticipated time during the OFF time by

blocking the ZCD pin of the converter, and enables the reflected voltage on the auxiliary

winding to be sensed after that time. This means that the controller has skipped one or

several valleys of the flyback voltage during the turn-OFF phases and has limited the

maximum switching frequency. The circuit’s time constant is set with capacitor C12 and

resistor R36. The limitations of the application’s frequency keep power losses on the

primary ESBT switch within reasonable values and contribute to the converter’s overall

stability. Refer to the L6565 datasheet for detailed information on the L6565 and function of

the circuit. All features, including the calculation of all setting components, are described in

STMicroelectronics’ application note AN1326 "L6565 quasi-resonant controller". This

application note also describes the ZVS concept.

The output voltage is controlled by a non-galvanic isolated primary feedback loop with

resistor dividers R38 and R42, and frequency response compensation components R39,

R40 and C14. All formulas related to the calculation of the frequency response

compensation during first trials and testing in this type of application are described in

STMicroelectronics’ application note AN2495 "80 W very wide input voltage range 3-phase

SMPS design based on L6565 and ESBT STC04IE170HV".

In SMPS applications, where the load can vary, the current of the primary switch also

fluctuates. To minimize power losses on the ESBT switch, the base current should be

proportional to the collector current or at least constant with the initial current overpeak of

the switching pulse - this is called the modifying envelope. It is important to avoid radical

over-saturation of the device at low loads and aim to optimize performance at full loads. To

achieve these driving requirements with a cost-effective solution, a simple driving circuit

providing a constant current into the base of the ESBT has been designed. This type of

solution is simple, cost-effective and minimizes power losses. The bias current for the base

of the ESBT is provided directly by the auxiliary power supply through the R22 resistor. For

a related base bias current, the value of this resistor is calculated according to the collector

current. According to the STC03DE220HV datasheet, for a maximum peak collector current

of 0.6 A, the gain and related base current should be 16 mA. During the storage time, when

the collector current for a certain period flows trough the B-C junction before this junction

recovers from conduction, the current flows into the C10 capacitor which stores some

energy and provides it again for the next switching cycle to create an initial base current

spike. Current that is not stored can also flow through D8 to the auxiliary supply area. Note

that during the storage time the collector current flows through the base and is stored in the

base capacitor C10, so that the quasi totality of energy is recovered. The R20 resistor limits

the inrush current floating from the C10 capacitor to the base of the Q2 transistor. In this

topology, the base current always has the same value and does not follow the variation of

the collector current, which appears while unloading the output. The constant base bias

current can cause over-saturation of the BJT structure in the ESBT in low- or no-load

conditions. However, in a case like this, the driver guarantees the appropriate switching of

the ESBT through the ZVS pin when the device is fully switched OFF and the carriers in the

BJT are fully recombined. This solution – which is very simple and cost-effective – implies

that the ESBT be driven with a constant current.

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