STCMB1 datasheet(45/59 Pages) STMICROELECTRONICS

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STCMB1

Application information

left-hand picture in Figure 12 on page 25. Once again, one MOSFET can be turned on while

the body diode of the other is conducting and this may happen for a few cycles.

This is automatically prevented with timeshift control. In fact, being the oscillator ramps

synchronized to the zero crossings of the tank current, a ramp in a half-cycle starts only after

the tank current has reversed in that half-cycle. As a result, the duty cycle of the initial cycles

is less than 50% until the dc voltage across the Cr reaches Vin/2, which eliminates the initial

V • s imbalance and ensures soft-switching. The picture of Figure 27 shows the start-up

waveforms of a resonant converter driven by the STCMB1.

7.24

HB: OCP management with internal digital delay and restart

In the STCMB1 the current sense input ISEN_HB pin senses the current flowing in the

resonant tank to perform two tasks:

Primary overcurrent protection (OCP function)

Hard switching cycle prevention

In this section the discussion is focused on the OCP function.

Unlike PWM controlled converters, where the energy flow is controlled by the duty cycle of

the primary switch (or switches), in a resonant HB the duty cycle is fixed and the energy flow

is controlled by its switching frequency. Thus, in resonant HB converters the most efficient

way to reduce an excessive current level is the increasing of the switching frequency.

In Figure 28 a couple of current sensing methods are illustrated that will be described here

following.

Figure 28. Current sensing techniques: a) with sense resistor, b) “lossless”, with capacitive shunt

In the circuit shown in Figure 28a, Rs is placed directly in series to the resonant tank. Its

value can be determined using Equation 27.

Equation 27

where IRES.pk is the maximum expected peak current flowing through the resonant capacitor

and the primary winding of the transformer, with the maximum load and the minimum input

voltage. The power dissipation in Rs can be approximately found with:

Equation 28

0.76V

IRES pk



---------------------

0.4IRES pk



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