HF PackerAmp V4, Construction Manual

Page: 45 / 76

HF PACKER
AMP
V4R6 Page 45
is mounted this way? It is done on purpose so that the thermal
mass of the circuit board traces will not unduly influence the
reaction time of thermal circuit breaker. It must get hot in order
to open the circuit. It will remain open until it cools down again.
So in that sense, it is self-resetting. It is designed so that it
takes a sustained 10A or more to trigger the circuit breaker.
There are no recorded cases where this part fails. In case of
reverse voltage, you will have to replace the diode with a
similar high current diode. We have spares.
DC-DC Converter, U4
The DC-DC Converter is operated on demand (when we
transmit) otherwise, it is off and silent during receive. If you
follow along by looking at the schematics page
PSU HFPA
10. There are only very few components to make this chip
work. We have the high current inductor, L4 50uH, the high
current series diode D4 and the two resistor R13 and R18. We
will touch on the controls a little later. TheLT1270A is a 10A
device. Meaning the device is rated for 10A current. It
operates under the principal that if you ground one end of the
inductor the current flowing through the inductor will attempt to
keep flowing at its current level when the ground is removed.
Pin 4 provides that ground. The inductor, following the laws,
will raise the voltage to near infinity to attempt to keep the
same current flowing. As a result, we get a step up of voltage.
On the anode side of D4, you will see a switching waveform
between ground and about 30V. On the cathode side you will
see a constant DC thanks to the electrolytic capacitors C16
and C21. So how do we regulate this beast? Glad you asked.
The resistors R13 and R18 form a voltage divider which sends
a small sample of the output voltage back to the U4 chip pin 2
(FB) or feedback pin. The sampled voltage on pin 2 is
internally compared to an internal voltage reference of 1.244V.
The U4 chip will adjusting the switching duty cycle of the
output on pin 4 to make the voltage on pin 2 equal to 1.244V,
the internal reference. So now the output voltage is set by the
ratio of R13 and R18 using simple ohms law principals. To
figure it out, you have 1.244V across R18, a 1.2K resistor.
That will set a known current flowing through R18. I=E/R or
1.224/1,200 or 0.001037A. This current is also flowing through
R13, the 27K resistor. So what is the voltage across R13?
E=I*R or 0.001037 * 27,000 = 28V. The voltage at the top of
R13 is equal to 1.244V + 28V or 29.25V. Your voltage may
vary due to the tolerance of the resistors and other subtle
variances. I find a typical of about 29.5V in several amps. If
you are trouble shooting this circuit you want to measure the
voltage on the tab of D4 (cathode). When the PSU is off, you
will essentially measure your battery input. When the PSU is
on, you will measure approximately 29.5VDC.
Timing Sequence of Control Signals
The PSU, IPS and TX are three control lines which are
synchronized by the Controller, U2. When RF input is
detected, the PSU is turned on first to get the 29.5V ready for
use. Next the TX line is switched from receive to transmit with
no power to MOSFETs at the time of switching. Next the IPS
line is asserted by U2 which operates the switch, U5 and
delivers the 29.5V to the drains of the MOSFETs. When RF
goes away, a reverse sequence is followed. First, the IPS
control line is made inactive which removes the 29.5V from the
MOSFET drains and then the PSU control line is made
inactive and finally the TX control is made inactive which
switches the AMP from transmit to receive. This can be
visualized (somewhat on the timing diagram shown on the