Design Concept for a Transformerless Solar Inverter

Vinotech Feature_Layout 1 02/02/2010 10:29 Page 28
Design Concept for a
Transformerless Solar Inverter
Vincotech is able to offer a wide spectrum of power modules for solar applications. For transformer-less
single phase solar inverter the power module FZ06BIA045FH-P897E is able to carry a output power of 6kW
but for efficiency optimisation a nominal power of 3kW is recommended. Michael Frisch and Temesi
Ernö, Vincotech Germany and Hungary
Figure 1: Single phase,
inverter topology
The topology is supporting the required
functions of adjustment to the maximum
power point (MPP) of the solar string and
inverting to sinusoidal output current and
voltage (see Figure 1).
The booster is active when the solar
voltage is below the peak of the power
grid voltage.
In this case the booster (T5, D8,9) sets
the MPP for the photo voltaic solar cell
(PV). When the PV MPP voltage reaches
the peak of the line, the bypass diode
(D7) cuts boost stage losses. The
adjustment of the MPP has to be
controlled by the output H-bridge inverter.
The output of the booster is the DC link
voltage, filtered by a capacitor (C4). This
capacitor should be a parallel composition
of a high tangent delta film capacitor and
an electrolytic capacitor. The high
frequency capacitor have to be placed
Issue 1 2010
Figure 2: Dual inductor
with split winding
Power Electronics Europe
Vinotech Feature_Layout 1 02/02/2010 10:29 Page 29
close to the module pins to limit overvoltage shoots at turn off of the MOSFET
(T2 and T4), while the electrolytic
capacitor should be sized for the 100Hz
power fluctuation of the 50Hz mains.
Figure 3: Wave form
dual inductor with
split windings
H-bridge inverter
The H-bridge works by asymmetric
unipolar modulation. The high side of the
asymmetric H-bridge should be driven by
50Hz half-wave dependent on the polarity
of the mains while the opposite low side is
PWM modulated to form the mains
sinusoidal shape.
The 10nF ceramic capacitor (C5) should
be placed close to the gate-emitter pins of
the high side transistors to eliminate cross
through conduction due to fast switching
of the low side transistors. A negative gate
turn off voltage on the high side gate may
also improve switching performance. The
low side gate drive resistor should be
selected to adjust the speed of MOSFET
Output filter and current sense
The inductors L1 and L2 are for the
differential mode (DM) and common
mode (CM) voltage filter. Both have a
double winding, one of each in both phase
connection (Figure 2).
However one of the inductors is
connected with opposite winding direction
in one phase connection. In this manner
the utilisation of the inductor becomes
more effective (Figure 3) than with single
winding (Figure 4) inductors and delta
capacitors, while still keeping the common
mode voltage noise between line and DC
link to an even lower level (Vcmd).
If the output current sense is put before
the inductor (L1), the test current will be
the sum of output current to grid and CM
(common mode) current to C1 and C2.
So two current senses have to be used
and put on the output line before L1. The
output current to grid is determined by the
sum of the two currents.
Power module
For a conclusive module design low
induction in the DC-link is a must. To
achieve this target, the internal inductivity
caused by wire bonding, layout and
module pinning has to be minimised. This
means the DC+ and DC- pins in the boost
circuit as well as in the output inverter
have to be placed as close to each other
as the standards allow. Also sense contacts
for the fast-switching power transistors are
The parasitic inductance of the wire
bond at switch on/off of the IGBTs or
MOSFETs will reduce the gate signal. This
might cause oscillations in the transistor or
at least increased switching losses. The
Power Electronics Europe
Figure 4: Dual
inductors with single
Figure 5: Wave form
of the dual inductor
single winding
Issue 1 2010
Vinotech Feature_Layout 1 02/02/2010 11:06 Page 30
current-less sense wire, bonded directly on
the source or emitter pad of the transistor
chip, will eliminate the problem. This is
only possible with module technology.
Figure 6 shows the Vincotech standard
module flowSOL0-BI (P896-E01) which
incorporates the functions listed previously
such as:
• Boost circuit with MOSFET
(600V/45m⍀) and SiC rectifier
• Bypass diode for maximum power
(when exceeding nominal power)
• H-bridge with 50A/600V IGBTs and
SiC rectifier in the high side and
MOSFET (600V/45m⍀) in the low
* Temperature Sensor
A simulation based on measured values
of this circuit (here are only the
semiconductor losses considered) shows
the following results (conditions PIN = 2kW,
fPWM = 16kHz, VPVnominal = 300V, VDC = 400V):
The efficiency for the module (booster +
inverter) is 98,8%. This shows that a total
efficiency, including the passive
components, of 98% is reachable. Figure
7 also shows that the efficiency of the
alternative full IGBT solution drops
significantly at partial load.
ABOVE Figure 6: Module
flowSOL0-BI incorporating
boost circuit and mixed
RIGHT Figure 7: Efficiency
simulation result for the
output inverter shows 99,2%
compared to 97,2% of a pure
IGBT solution (dotted line)
To receive your
own copy of
subscribe today at:
Issue 1 2010
Power Electronics Europe