L6599 STMicroelectronics, L6599 Datasheet - Page 23

• Manufacturer Part Number: L6599
• Description: High-voltage resonant controller
• Manufacturer: STMicroelectronics

Specifications of L6599

• Dimensions (l/w/h): 132 mm / 52 mm / 22 mm
• Universal Ac Mains Range: 90 VAC to 264 VAC
• High Output Power: 90 W (4.74 A, 19 VDC)
• Very Low Output Noise And Ripple: less than 300 m VP-P
• Very Low Standby Power: less than 0.5 W at 264 V
• Hiccup Mode: Auto recovery with SCP
• Latch Mode: OVP acts while VOUT > 29 VDC

L6599
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
described in the following. The circuit of
resistor Rs might not be negligible, hurting efficiency; the circuit of
complex but virtually lossless and recommended when the efficiency target is very high.
Figure 30. Current sensing technique with sense resistor
Figure 30 and Figure 31
L6599
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a couple of current sensing methods are illustrated that will be
ISEN
τ
≈
f
10
min
Figure 30
Rs
Cr
is simpler but the dissipation on the sense
I
Cr
Vspk
0
Application information
Figure 31
is more
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