Ecleree PAM2600, Руководство по обслуживанию

Страница: 7 / 107

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