JBL A6000GTi, Service Manual, Page: 14 / 62

137159-1 Rev. 1 A6000GTi Service Manual

2003 Crown Audio, Inc.
Theory Of Operations 3-3
3.1.5. Output
The PWM modulated 125kHz signals exit the modulators and enters a voltage translator
formed by Q301
– Q310. This string translates the balanced PWM signals into a single
ended waveform with increased amplitude for the FET driver (U301 and U302). The FET
driver’s output is increased by a two transistor discrete amplifier stage for improved fan out
capability.
The FET driver (U301) output is referenced to Vp+14 and Vp. The Vp+14 floating supply
is derived from +Vcc on the main PWA. Therefore, it is proportional to Vcc and varies as
the +/- Vcc levels vary.
We pause now for a qualitative description of output stage operation (refer to Figure 3.1).
All parts to the left of “Iout” are positive or “p” side parts and all parts to the right of “Iout”
are negative or “n” side parts. The switches Swp and Swn are analogous to the FETs.
Quiescent operation will be considered first. At the start of a switching cycle Swp and Swn
are both turned on. Current flows from +Vcc, through Swp, through Lp, through Ln,
through Swn, and down to
–Vcc. The current rises at a controlled rate in the Lp and Ln
inductors (see the current diagram to the right in Figure 3.1). Halfway through the cycle,
the switches turn off but the inductors have reached a certain current flow (the peaks on
the diagram) and now must continue pushing current in the same direction. The current
continues to flow in the same direction through the inductors but comes through the diodes
because the switches are open. Specifically, current flows from
–Vcc through D1, through
Lp, through Ln, through D2, and to +Vcc while ramping down. At quiescent, the Lp and Ln
currents cancel so the net voltage developed at Iout across the capacitor is zero. For
positive voltage output, Swp is left on longer than Swn and for a negative output voltage,
the opposite occurs. Note that for any output voltage, the “on” time of both switches will
overlap, even if one is on longer than the other.
Figure 3.1 BCA Operation