Ecler PAM1400, Service Manual

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4
This resistance performs a negative feedback on the gate, lowering down the Vgs, relating to the drain current;
like this:
Vgs = Vgg - Id*Rs
The higher the Id, the lower the Vgs voltage. The gate is
protected by a zener, preventing a possible overload during an
unexpected change from overload to real clipping.
Given the high input impedance and the broad frequency response of the MOSFETs there is a high risk of
self-oscillations if all gates are excited connected to the same node. Intercalating serial resistances and ferrite beads at
the gate this possibility is minimized, because the Q of the LC network made by the inductances and gate-source
capacity is reduced.
PROTECTION CIRCUIT
The protection circuit monitors the dissipated power at the MOSFETs stage. It has two basic parts:
MOSFET Id current detection.
MOSFET Vds voltage detection.
The goal is limiting the MOSFET so it works inside an area close to the SOA, as indicated in the figure. We
chose channel N because, due to construction reasons, its SOA is lower.
ZONE A. This zone is for very low loads, around 0 Ω . As the load voltage is very low, the voltage held by the
MOSFET will always be high. The protections should be activated with very low current.
Fast protections and some of the slow ones are working in
this zone. The circuit that configures the fast ones is made of: D120,
D121, D123, R174, R175, R176, R177, R178, R179, C127, Q122
and Q123 for the N channel. There is also an equivalent circuit in the
P channel. These start working when there is a sudden current
variation because of a shortcircuit or a transitory. The reaction time
-from the exact moment when these things occur to when the current
stops circulating through the MOSFETs- is about 80
µ s.
The time constant is given by C127, R174 and R179 and the
load circuit made by the LED diode of the IC104 (opto-coupler).
Please note that in order for the protection to be activated
Q122 and Q123 must conduct simultaneously, through which R174
is linked to negative power supply, being C127(1µF) loaded very quickly through this resistance, activating the LED of
the opto-coupler, sending a pulse to the protection circuit, which will open the corresponding channel's relay, being this
way the output from the power amplifier disconnected from the load (0
Ω ), in this case. Q122, together with the zeners
and the base polarization resistances, configure the voltage detector (this group is in parallel with the Vds voltage of the
N MOSFET).
Id
C B A
Vds