AN4407
Application note
Advantage of the use of an added driver source lead in discrete
Power MOSFETs
Antonino Gaito, Marc Laudani, Massimo Nania, Cristiano Gianluca Stella
Introduction
In modern power supply design, more and more attention is given to the electrical efficiency
of an overall system and to the junction temperature of semiconductor devices handling all
the power to be converted into a usable form.
Among all the semiconductor devices, transistors are by far the most important category;
almost all of them are three pin devices (MOSFET, BJT, IGBT) and, as opposed to diodes,
they have a driving section which makes them more sensitive to issues related to the
interaction between power to handle and input signal. Even if this document focuses on
Power MOSFETs, in some cases the results could be extended to other power transistors
depending on the current level to be switched and on the switching speed. The aim of this
document is to illustrate the limitation related to a 3-pin device and detail the advantages of
using a fourth driving source pin, also known as Kelvin source, besides the traditional power
source.
In a 3-pin device, during every switching (turn-on or turn-off) cycle, the stray inductance of
the source wire bonding coupled with the slope of the current being interrupted always
generates a voltage signal opposite to the driving signal (V
GS
) of a MOSFET, in our case.
The effect of this opposing signal is to slow down the switching cycle, which in turn
increases the cycle by cycle switching loss. The introduction of the Kelvin source pin allows
separating the path of the power from that of the driving signal and to refer the driving
network to the Kelvin source, where no current is supposed to flow. In this situation, it is
possible to obtain a driving signal which is immune to any disturbance deriving from the
verylarge and fast current variation flowing though the power path. This results in the
reduction of the overall power loss in the transistor, and consequently a lower operating
temperature and potentially more reliable and longer lasting power systems.
January 2015
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www.st.com
Contents
AN4407
Contents
1
Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1
1.2
Approach to the study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Parasitic inductance effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.2.1
1.2.2
Turn-off . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Turn-on . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.3
Comparison analysis between 3 pins and 4 pins devices in efficiency and
temperature; turn-on and turn-off energy at different output power . . . . . 12
2
3
4
5
Eon / Eoff energy: experimental waveforms . . . . . . . . . . . . . . . . . . . . . 15
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
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Description
1
Description
The aim of the following paper is to illustrate the analysis evaluating the performance of the
STMicroelectronicsMDmeshM5 super-junction MOSFET housed in a TO-2474-lead
package with an additional driving pin.
The new electrical symbol and package are shown in
Figure 1.
Note the repositioned drain,
source and gate pins to allow the introduction of the Kelvin pin (pin 3), always ensuring the
right creepage distance.
Figure 1. Package silhouette and electrical symbol (4 pin)
Drain(1)
Gate(4)
1
TO247-4
2
4
3
Driver
source(3)
Power
source(2)
GIPG211120131627FSR
A demonstration power board (Figure
2)
was created for test purposes, which implements a
2 kW PFC designed for industrial applications, using a FOT (Fixed-Off-Time) control
patented by ST. The PFC stage functions mainly in CCM and can be supplied directly from
AC mains between 185 and 265 V
RMS
to provide a 400 V stable and regulated DC bus. This
system was chosen because it allows investigation of the parasitic effect on the switching
system. Furthermore, the working CCM properties allow investigation of both the on and off
switching operations.
Table 1shows
the principal electrical specifications.
Table 1. Electrical characteristics
Parameter
V
in
Line frequency
Output voltage
Max output voltage
Output voltage ripple
Max switching frequency
Ripple (Kr) inductor current
IC in used
Value
from 185 V
ac
to 265 V
ac
50 Hz
400 V
dc
2000 W
5%
55 kHz
0.25
L6563
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Description
Figure 2. Demo board of PFC 2 kW using MOSFETs in TO247-4 (4 pin)
AN4407
GIPG211120131727FSR
1.1
Approach to the study
Usually, in electronic power systems, a feedback network is implemented to create a control
of the electrical parameters (for the DC/DC converter, this can involve both the output
voltage and output current or only one of these). In order to control the current on the
system, a resistor is often connected in series to the line under control, forming a common
path between the input and output network. The information coming from this element,
(voltage across the sensing resistor) is used as the control signal and sent to the pin of the
driver. This pin is connected to an input of the internal comparator inside the driver; it’s
output signal is used to regulate the conduction time of the power switch.
When the power switch requires particular source/sink current values, the drivers are
created with a push-pull on the output (Figure
3).
The feedback type is known as current
sampling series-mixing, which offers the main benefit of increasing the input impedance,
while the drawback lies in the impact of the output net on the driving circuitry. A possible way
to reduce the latter phenomenon is to separate the input circuit from the output one, which is
along the lines of the solution proposed by STMicroelectronics.
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Figure 3. Schematic of a push-pull driver for power device
Description
GIPG211120131731FSR
A current amplifier solution can be integrated directly inside the driver or put in series with
the output driver externally. This solution is used on the demo board made by
STMicroelectronics. Since the L6563 driver used on the demo needs a sensing signal for
current control, a resistor is put in series with the source of the power device (Figure
4).
Figure 4. Schematic of a push-pull connection usingan L6536 as the driver
GIPG211120131734FSR
This solution necessitates a slight modification on the driving network to correctly use the
new device with the added sensing pin. In fact, if we directly replace the standard 3-pin TO-
247 MOSFET with the 4-pin version (TO-247-4), we risk short-circuiting the sensing resistor
and losing any information coming from it (Figure
5).
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