Application information

L6599A

22/35

Doc ID 15308 Rev 7

Equation 5

quite empirical and is a compromise between an effective soft-start action and an effective

OCP (see next section). Please refer to the timing diagram of

Figure 27 to see some

significant signals during the soft-start phase.

7.4

Current sense, OCP and OLP

The resonant half bridge is essentially voltage-mode controlled; therefore a current sense

input only serves as an overcurrent protection (OCP).

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

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

is controlled by its switching frequency. This impacts on the way current limitation can be

realized. While in PWM-controlled converters energy flow can be limited simply by

terminating switch conduction beforehand when the sensed current exceeds a preset

threshold (this is commonly known as cycle-by-cycle limitation), in a resonant half bridge the

switching frequency, that is, its oscillator frequency must be increased and this cannot be

done as quickly as turning off a switch: it takes at least the next oscillator cycle to see the

frequency change. This implies that, to have an effective increase able to change the energy

flow significantly, the rate of change of the frequency must be slower than the frequency

itself. This, in turn, implies that cycle-by-cycle limitation is not feasible and that, therefore,

the information on the primary current fed to the current sensing input must be somehow

averaged. Of course, the averaging time must not be too long to prevent the primary current

from reaching too high values.

In

Figure 28 a couple of current sensing methods are illustrated and are described in the

following. The circuit of

Figure 28a is simpler but the dissipation on the sense resistor Rs

might not be negligible, damaging efficiency; the circuit of

Figure 28b is more complex but

virtually lossless and recommended when the efficiency target is very high.

SS

3

SS

min

start

min

SS

R

10

3

C

;

1

f

f

RF

R

−

⋅

=

−

=