L6599AT

Application information

Doc ID 15534 Rev 3

19/31

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 10 to see some

significant signals during the soft-start phase.

6.4

Current sense, OCP and OLP

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

only serves as 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 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, capable of changing 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 excessively high values.

In Figure 11, a few current sensing methods are illustrated that are described in the

following paragraphs. The circuit in Figure 11a is more simple, but the dissipation on the

sense resistor Rs may not be negligible, decreasing efficiency. The circuit in Figure 11b 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

−

⋅

=

−

=