Plasmatronics PL20 - PWM and Slow Switching

Rev 6.3.0 12.09.16
PL Reference Manual
27
SET/MODE/PWM: [PROG= 4 Only]
Selection Function
0 No PWM used
1 PWM on SOL- terminal only
2 PWM on LOAD- terminal only
3 PWM on both
7
SOL- input (1st solar) and switch all the other
panels (2nd solar) across with the relay . This
leaves a regulated ‘maintenance’ charge available
for the primary battery, and can minimise the
amount of switching required by the relay (which
switches over each time the primary battery bank
voltage falls below FLTV - HYST). The maximum
relay switching rate =30 seconds.
Consider: Are you sure that this setup is the
best use of available charge current? Consider
if it would be better to increase your primary
battery bank.
7.5.5 PWM and Slow Switching
When the PL is trying to keep the battery voltage
constant, it does this by turning the charge
current on or off as required. It can do this
quickly or slowly.
PWM (fast switching):
In the fast switching mode, the PL uses pulse
width modulation (PWM) at 125Hz. It is
recommended that PWM control be used unless
there is good reason not to, as it provides much
smoother control and faster charging.
PWM may generate some audio or radio
frequency interference. If you experience noise on
electronic equipment (for example AM radio), you
could try selecting SET/MODE/PWM = 0 (slow
switching only, option available when PROG=4).
If this does not fix the problem, it’s unlikely that
the noise is generated by the PL controller.
The radio frequency interference from the PL has
been tested in PWM mode and found to be very
low. It is at least 15dB below the domestic limit for
European and Australian EMI compliance standards.
NOTE: More care must be taken at higher system
voltages (particularly 48V) because the inductance
of the wiring to the photovoltaic panels can
cause some ‘ringing’ on turn-off. Keep wiring as
short as possible and panel cable pairs together
(cable tie). On long wiring runs at higher voltages,
some damping may be necessary and possibly
some external clamping of transients. With highly
inductive wiring, the transients may be large
enough to damage the FET switch. Consult the
factory if further information is required.
LOAD- terminal PWM switching:
For PWM control to be available on the LOAD-
terminal, LSET must be set for Function 10
(Diversion Control). DO NOT set LSET to
Function 11 (Inverted Diversion Control) if
using PWM on load terminal—see the LSET
table.
PWM should only be enabled on the LOAD-
terminal (PWM = 2 or 3) if the PL is using the
LOAD- terminal to directly switch a dump load
(no relay).
An exception to this is when the LOAD-
terminal is driving a solid state ‘electronic’ relay
(non-mechanical), as these can usually switch at
PWM speed (125 Hz).
NOTE: PWM is not available on the ‘G’
terminal.
Slow Switching (non PWM):
In slow switching (non PWM) mode, the PL
controls the battery voltage by slowly switching
the charge current on and off. It turns the
charge current OFF at the voltage set point for
the relevant charge state, and turns the charge
current back ON at a slightly lower voltage.
The gap between these two voltages is called
hysteresis and is adjustable (HYST). The larger
the hysteresis value, the slower the rate of
switching on and off will be.
HINT: Increase Hysteresis value if you are using
diversion control with a mechanical relay so the
relay is switching less often.
In the slow switching mode, the PL limits the
maximum switching speed to 0.25 seconds
between changes. This virtually eliminates any
audio or radio frequency interference.