L6599 datasheet(23/36 Pages) STMICROELECTRONICS

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L6599

Application information

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7.4

Current sense, OCP and OLP

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

will only serve 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 now as cycle-by-cycle limitation), in a resonant half-bridge the

switching frequency, that is, its oscillator's 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 29 and Figure 30 a couple of current sensing methods are illustrated that will be

described in the following. The circuit of Figure 29 is simpler but the dissipation on the sense

resistor Rs might not be negligible, hurting efficiency; the circuit of Figure 30 is more

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

Figure 29. Current sensing technique with sense resistor

L6599

ISEN

τ ≈

fmin

Vspk

ICr

L6599

L6599 Datasheet(PDF) 23 Page - STMicroelectronics