Application Note Small Signal OptiMOS™ 606 MOSFET in Low Power DCDC converters

Application Note AN 2012-12
V2.0 December 2012
Small Signal OptiMOS™ 606 MOSFET in
Low Power DC/DC converters
IFAT PMM APS SE DS
Pradeep Kumar Tamma
Small Signal OptiMOS™ 606 MOSFET in
Low Power DC/DC converters
Application Note AN 2012-12
V2.0 December 2012
Edition 2012-12-06
Published by
Infineon Technologies Austria AG
9500 Villach, Austria
© Infineon Technologies Austria AG 2011.
All Rights Reserved.
Attention please!
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PROPERTY RIGHTS OF ANY THIRD PARTY) WITH RESPECT TO ANY AND ALL INFORMATION
GIVEN IN THIS APPLICATION NOTE.
Information
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Small Signal OptiMOS™ 606 MOSFET in
Low Power DC/DC converters
Application Note AN 2012-12
V2.0 December 2012
Table of contents
1 Introduction .................................................................................................................................................. 4
2 Application Example ................................................................................................................................... 4
2.1
Application Boundary conditions ........................................................................................................ 5
2.2
Application circuit configuration ......................................................................................................... 5
3 Review of Losses in a DC/DC converter ................................................................................................... 6
3.1
MOSFET Switching Losses ............................................................................................................... 6
3.2
MOSFET Gate Losses ....................................................................................................................... 8
3.3
MOSFET Output Losses .................................................................................................................... 8
3.4
MOSFET Conduction Losses ............................................................................................................ 8
4 Calculation of MOSFET Losses in the application................................................................................... 9
5 Comparison and Summary .......................................................................................................................11
3
Small Signal OptiMOS™ 606 MOSFET in
Low Power DC/DC converters
1
Application Note AN 2012-12
V2.0 December 2012
Introduction
™
Infineon’s new 60V class Small Signal OptiMOS 606 family will be available in TSOP-6, SOT-89 and SC59
packages.
The low Qg and low RDS (on) makes the OptiMOS™ 606 suitable for low power DC/DC converters and cell
balancing in Battery Energy Control Modules (BECM). Also the logic level gate enables it to be easily
interfaced directly with MCUs / Digital circuits.
As all products are qualified to AEC Q101, they are ideally suitable for automotive and high quality
demanding applications.
This application note illustrates the benefits of BSL606SN (TSOP-6 package) in low power DC/DC
applications. The main features of BSL606SN are shown in the table below.
Parameter
Symbol
Conditions
Value
Unit
Continuous drain current
ID
TA = 25 °C
4.5
Drain-source breakdown voltage
V(BR)DSS
VGS =0 V, ID = 250 µA
60
-
-
Gate threshold voltage
VGS(th)
VDS = 0 V, ID = 15 µA
1.3
1.8
2.3
Drain-source on-state resistance
RDS(on)
VGS =4.5 V, ID = 3.6A
69
95
mΩ
Gate to source charge
Qgs
1.9
2.5
nC
Gate to drain charge
Qgd
VDD = 48 V, ID = 4.5 A,
VGS = 0 to 5 V
1.0
1.5
Gate charge total
Qg
4.1
6.1
A
V
Table 1: BSL606SN Main Features
2
Application Example
A good example of a low power DC/DC converter is an LED power supply. In Automotive, LED lighting is
now common. A typical automotive lighting application is the Daytime Running Light (DRL) function such as
the one shown in the picture below.
Figure 1: DRL function example
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Small Signal OptiMOS™ 606 MOSFET in
Low Power DC/DC converters
2.1
Application Note AN 2012-12
V2.0 December 2012
Application Boundary conditions
An example of the application boundary conditions for a DRL function is given below.

The sum of the LED forward voltage (DC/DC converter output voltage VOUT) is approximately 25 V.

The supply input voltage VIN is specified in the range of 8 V to 16 V. The nominal value is 12 V.

The LED current or the output current of the DC/DC converter I OUT should be 400 mA.

Boost configuration is used with a switching frequency of around 400 kHz operated in continuous
conduction mode.
2.2
Application circuit configuration
The figure below illustrates the Boost configuration of a DRL application.
Figure 2: Boost circuit configuration
A summary of the application boundary conditions, which should be the basis for the calculation of the losses
in the application, are presented in the table below
Symbol
Value
Unit
Name
VIN
12
V
Nominal input Voltage
VOUT
25
V
LED forward voltage
IOUT
0.4
A
LED Current
Fs
400
kHz
Switching Frequency
ΔVOUT
100
mV
Max. ripple voltage on VOUT
ΔIL%
20
%
Pk-Pk inductor Ripple current.
Table 2: Application Boundary conditions
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Small Signal OptiMOS™ 606 MOSFET in
Low Power DC/DC converters
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Application Note AN 2012-12
V2.0 December 2012
Review of Losses in a DC/DC Converter
The efficiency of a DC/DC converter is a measure of the ratio of the output power supplied to the load with
respect to the input power. The input power is equal to the load power plus the converter losses. A DC/DC
converter has its losses in its control circuit and magnetics, out of which switching losses are the greatest
contributor. These losses are briefly discussed in the following sub-sections.
3.1
MOSFET Switching Losses
Switching losses occur due to the positive product of current through the MOSFET and voltage across it
during switching transition. The switching losses occur twice for every switching period during turn-on and
turn-off. The figure below illustrates current and voltage during turn-on.
Figure 3: MOSFET turn on waveforms
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Small Signal OptiMOS™ 606 MOSFET in
Low Power DC/DC converters
Application Note AN 2012-12
V2.0 December 2012
Using linear approximations of the waveforms, the power loss components for the respective intervals can
be estimated. The estimated power components in intervals T2 and T3 are given by:
ID
 Fs
2
(1)
VDS
 I D  Fs
2
(2)
P2  T2  VDS 
P3  T3 
The total turn-on switching losses are the sum of the two components and are given by:


1
1
Pon  P2  P3   T2  T3  VDS  I D  Fs   TSW (on)  VDS  I D  Fs
2
2
(3)
Where TSW (on) is the turn-on time for the MOSFET it depends upon the gate drive voltage.
The average gate drive currents during T2 and T3, IG, T2 and IG, T3 are given by:
I G,T 2 

Vcc  0.5 V pl  Vth

(4)
Rtot
I G,T 3 
Vcc  V pl
(5)
Rtot
Vcc = gate driver power supply. Rtot = total gate resistance.
Assuming that Ig, T2 charges the input capacitors of the MOSFET from Vth to Vpl and Ig,T3 is the discharge
current of the gate to drain capacitor while the drain voltage changes from VDS to 0 then the approximate T2
and T3 are given by:
T2  Ciss
V pl  Vth 

T3  Crss 
(6)
I G ,T 2
(7)
VDS
I G ,T 3
The turn-on time, TSW (on) is the sum of T2 and T3 and is given by:
TSW (on)
Ciss  V pl  Vth  Crss  VDS
 T2  T3 

I G ,T 2
I G ,T 3
(8)
Similarly the turn-off switching losses can be estimated and are given by:
(9)
1
Poff   TSW ( off ) VDS  I D  Fs
2
Thus the total switching losses are the sum of turn-on losses and turn-off losses.
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Small Signal OptiMOS™ 606 MOSFET in
Low Power DC/DC converters
Psw  Pon  Poff 
Application Note AN 2012-12
V2.0 December 2012
1
 TSW ( on)  TSW ( off )   VDS  I D  Fs
2
(10)
Where, TSW (off) is the turn-off time for MOSFET.
3.2
MOSFET Gate Losses
Energy is required in order to charge and discharge the gate capacitances of the MOSFET for each
switching period. This energy is usually dissipated through gate series resistance in the gate driver circuit.
Thus the gate losses are given by:
Pg  Qg  Vcc  Fs
(11)
QG = total gate charge, Vcc = gate driver power supply voltage and Fs = switching frequency of the converter.
3.3
MOSFET Output Losses
The output losses are the energy losses when the MOSFET output drain to source capacitance is
discharging during turn-on. Thus the output losses are given by:
(12)
2
Pout  1  CDS  VDS  Fs
2
CDS = MOSFET output Drain to Source capacitance.
3.4
MOSFET Conduction Losses
In the on-state, MOSFETs do not behave like an ideal switch with zero impedance, but instead they have a
small resistance typically called RDS (on). Due to this resistance there will be power losses in the MOSFET and
these are calculated by:
Pcond  RDS ( on)  I D,RMS
(13)
2
Ids, RMS = RMS value of the current through the MOSFET.
The RMS values can be calculated by the current waveform in the MOSFET. When the converter is
operating in continuous conduction mode the MOSFET current wave form will be as shown in the figure
below. In the figure, IL, avg is the average current in the inductor, Ts is the switching period, D is the duty ratio
and ΔIL is the ripple current in the inductor
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Small Signal OptiMOS™ 606 MOSFET in
Low Power DC/DC converters
Application Note AN 2012-12
V2.0 December 2012
Figure 4: Current through MOSFET
When the MOSFET is turned on, the current within it is equal to the current in the inductor. Thus the RMS
value of the MOSFET current is given by:
I D ,RMS
(14)

(I L ) 2 
2

 D. I L ,avg 
12 

So the total MOSFET losses will be the sum of all losses
PMOSFET  Psw  Pg  Pout  Pcond
4
(15)
Calculation of MOSFET Losses in the Application
From the above section, BSL606SN losses can be estimated using the specified application values from
Table 2.
Assuming the MOSFET is driven by a 5 V gate drive power supply with 10 Ohms of total gate resistance and
the input voltage is 8 V the following is the case.
The switching losses of BSL606SN are:
Psw 
Psw 

1
 TSW ( on)  TSW ( off )   VDS  I D  Fs
2

1
 4.5  10 9 s  0.15  10 9 s  25V  0.4 A  400  10 3 Hz  0.009W
2
9
(16)
Small Signal OptiMOS™ 606 MOSFET in
Low Power DC/DC converters
Application Note AN 2012-12
V2.0 December 2012
TSW (on) and TSW (off) are calculated as discussed in section 3.1.
The gate losses of BSL606SN are:
Pg  Qg  Vcc  Fs
Pg  3.8 10 9 C  5V  400 103 Hz  0.008W
(17)
The output losses of BSL606SN are:
2
2
Pout  1  CDS  VDS  Fs  1  Coss  Crss   VDS  Fs
2
2


1
1
2
2
Pout   180 10 12  1110 12  25V   400 10 3 Hz  169 10 12 F  25V   400 10 3 Hz  0.02W
2
2
The conduction losses of BSL606SN are:
P
R
I
cond
DS (on) DS , RMS
I DS , RMS
2
2
2


I L 

0.24 A
2
2


 D. I L ,avg 
 0.68   1.25 A 
12 
12


For the boost converter the duty ratio D 
VOUT  VIN 
VOUT

  1.03 A


and the average current through the inductor is
equal to the average input current for the converter and is given by
I IN ,avg  I L ,avg 
I OUT
and the
1  D 
inductor ripple current ΔIL is given in table 2.
2
P
 0.066  1.03 A  0.07W
cond
(19)
10
(18)
Small Signal OptiMOS™ 606 MOSFET in
Low Power DC/DC converters
Application Note AN 2012-12
V2.0 December 2012
Thus the total losses of BSL606SN are summarized in the table below
BSL606SN
Conduction Losses
Pcond
70
mW
Switching Losses
Gate Losses
Output Losses
Total MOSFET losses
Psw
Pg
Pout
09
08
20
108
mW
mW
mW
mW
PMOSFET
Table 3: BSL606SN losses in application
5
Comparison and Summary
BSL606SN losses are compared with an automotive qualified competitor part of the same voltage class. The
figure below illustrates the different losses in the respective device.
Total Losses
180
160
Losses (mW)
140
120
Poss
100
Pdrv
80
60
Psw
40
Pcon
20
0
BSL606SN
competitor
part
Figure 5: Loss comparison
The chart shows that there are significantly less losses in the converter when BSL606SN is used in
comparison to that of the competitor part.
The benefits of using BSL606SN in low power DC/DC converters can be summarised as follows:

The gate losses are comparitively very small in BSL606SN. This means that the gate requires little
current even at logic level to turn it on. This makes it easy to interface to MCUs/ digital circuits.

The switching losses are very small with BSL606SN. This gives the flexibility of increasing the switching
frequency which in turn increases the transient performance. With the increase in switching frequency
the inductance can be decreased which in turn decreases the total converter size and cost.
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