Current sensing, OCP and OLP

L6699

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Doc ID 022835 Rev 2

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Current sensing, OCP and OLP

In the L6699 the current sense input ISEN (pin 6) senses the current flowing in the resonant

tank to perform multiple tasks:

1.

Primary overcurrent protection (OCP function).

2.

Hard-switching cycles prevention at startup (see

Section 6.3: Safe-start procedure).

3.

Capacitive-mode detection during operation (see

Section 9: Capacitive-mode detection

function).

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

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 has an impact on the way current limitation can

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

terminating switch conduction in advance when the sensed current exceeds a preset

threshold (cycle-by-cycle limitation), in a resonant half bridge the most efficient way to

reduce an excessive current level is to increase the switching frequency, i.e. the oscillator

frequency.

In

Figure 18 and 19 a couple of current sensing methods are illustrated.

Note:

The L6699 must sense the instantaneous tank current for proper operation of the smooth

startup function and the capacitive-mode detection circuit. Therefore, if a smoothing RC

circuit (the one shown in the dashed box) is used to reduce the noise level on the ISEN pin,

its time constant RF CF should be in the range of 100 - 200 ns. With slightly longer time

constants it is recommended that converter operation close to the capacitive-mode

boundary and during short-circuit, as well as at the end of the smooth-start phase, be

checked for possible hard-switching cycles. With considerably longer time constants (> 200

ns) hard-switching under the above mentioned conditions becomes very likely.

Figure 18.

Current sensing techniques

with sense resistor

Figure 19.

Current sensing techniques

“lossless”, with capacitive

shunt