Tait TB8100 Service Manual page 209

EMC Filter and
Protection
DC Power Converter
(DCDC)
DC Control
Hysteresis Mode
Battery Control
TB8100 Service Manual
© Tait Electronics Limited September 2006
The DC input is fed first to an EMC filter consisting of bus bars, an e-core
transformer, and capacitors. These components are located on the DC
input filter card, which is connected to the DC converter board via the bus
bars and soldered tabs.
The protection is provided by an anti-parallel diode, which prevents damage
from reverse polarity input voltage. External series protection (e.g. fuse or
circuit breaker) must be provided by the user.
In this stage the filtered DC input is fed to a push-pull converter which
converts the DC input voltage to a regulated DC output voltage.
The DC voltage is fed to a high frequency transformer which provides
galvanic isolation between input and output via two pairs of power
MOSFETS on the primary winding.
The output from this transformer is rectified and filtered before being fed to
the current sense and output filter circuitry on the AC converter board.
This secondary control circuitry consists of a voltage amplifier, which
provides a reference voltage and error signal. This error signal is fed via an
opto-coupler to the PWM controller IC, which is part of the primary
control circuitry.
The power supply for the DC control circuitry is provided by the battery
control card. The DC control circuitry is located on a separate card which
plugs into the DC converter board.
When the DC control circuitry receives a "hysteresis enable" signal from the
microprocessor, it changes the regulation to a hysteric method (hysteresis
mode). Hysteresis mode is used in sleep mode to improve the efficiency of
the DC converter under light loads. Sleep mode is configured with the
Service Kit and can only function if the load on the PMU is less than 10W.
In hysteresis mode the DC converter is switched off when the output
voltage reaches the maximum threshold, and on again when the voltage
reaches the minimum threshold. The thresholds are set in hardware at the
factory.
While the DC converter is switched on, the output capacitors are charged.
When the DC converter is switched off, the output capacitors are
discharged by the load connected to the PMU. The discharge time varies
in proportion to the load. The longer the discharge time, the greater the
efficiency of the PMU, as less power is drawn from the battery.
The battery control circuitry monitors the DC input voltage from the
battery. If the voltage is within the specified range, power is supplied to the
control circuitry and standby power supply (if fitted). Protection is provided
against the wrong input voltage being supplied.
Power Management Unit Circuit Description 209