AN2844

Adapter features

9/37

ESBTs with this type of configuration offer very good performance in terms of power losses,

and have a low cost compared to other available switches such as very high voltage power

MOSFETs. For further information on driving networks, refer to STMicroelectronics’

application note AN2454 "Universal input voltage power supply for ESBT-based breaker and

metering applications".

The main T1 transformer used is a layered-type transformer, which uses a standard ETD29

core with a bobbin. The ETD29 bobbin has been chosen because of its strong voltage

isolation capacities at such high input voltages. In terms of just power requirements, an even

smaller core area than the ETD29 could be used. A sandwich topology has been used for

the design of the winding, offering better coupling of windings compared to standard

topologies with only one primary winding.

This transformer has been designed according to STMicroelectronics’ released application

notes, with a flyback voltage of 250 V. The turn ratio between the primary and secondary

side has been calculated and is approximately 70. Refer to AN1326 "Quasi-resonant

controller" and AN2495 "80 W very wide input voltage range 3-phase SMPS design based

on L6565 and ESBT STC04IE170HV" for all necessary calculations.

As is common in flyback applications, the total voltage across the switch can reach very high

voltages. The calculation is done with the formula:

maximum overvoltage allowed by the clamping network and has been fixed to 200 V.

Allowing for some margin, a related switch STC03DE220HV with a breakdown voltage of

2200 V fills the requirements for these types of application.

A clamp network is used for leakage inductance demagnetization. In this particular case,

a C1 capacitor with related passive resistors R2, R3 and blocking diodes D2 and D5 used in

series because of voltage stresses, has been selected for this purpose.

The secondary side comprises a Schottky barrier diode D3 as rectifier, and filtering

capacitors C3 and C4 featuring low serial resistance. The short-circuit protection features for

the converter have been designed with transistors Q7 and Q6 and related passive parts.

The Q7 transistor senses the output voltage through the resistor dividers R38 and R42. In

normal conditions, the Q7 transistor keeps the Q6 transistor turned off. During a short-circuit

condition where the output voltage is very low or equal to zero, the Q7 transistor is closed.

Energy stored in the tank capacitor C9 can start to provide the supply current for Q6, which

starts to block the function of the converter through the L6565’s ZCD pin. This condition

continues until all the energy from the C9 capacitor has been discharged. The time cycle is

set with the R37 resistor and capacity of C9. Once all the energy in C9 has been

discharged, the converter starts to work again. If the short connection on the output is still

present, the short-circuit protection repeats until the short circuit is removed.