Application Note 1816
May 2011
www.microsemi.com
1/3
Device Selection and Optimizing of Half Bridge RF Generators
George J Krausse III, Vice President, MOS RF
Microsemi Power Products Group
405 SW Columbia St., Bend, OR 97702 USA
gkrausse@microsemi.com
The following Application Note is a SPICE Model tutorial on Half Bridge Topologies and device selection. We will
also discuss their relative performance with matched and reactive loads. This tutorial is based on new Large Signal
RF Spice Models developed at Microsemi PPG. The circuits include all significant strays at appropriate values. This
format allows us to explore operational elements that are exceedingly difficult, if not impossible, to measure
accurately even in a laboratory environment. In addition, it provides a valuable tool for the understanding of circuit
operation under varying load conditions and device type.
N Channel – N Channel Half Bridge
The Half Bridge Topology is used for this Device Characterization. Figure 1 illustrates the classical N Channel - N
Channel Half Bridge RF Generator. The High Side Switch X2 and the Low Side Switch X1 form the two active
devices in the Half Bridge. X1 and X2 commutate in an alternating fashion providing a pseudo Square Wave drive
to the input of the RF Network, at V5. The RF network provides an impedance match from the Drain Impedance of
X1, X2 of Figure 1. The match is 3Ω to the 50Ω load. During the evaluation, different devices will be examined.
This will necessitate changes in the network in order to provide an appropriate drain load match. This L Match
network is resonant at the frequency of the device evaluation.
This resonant network only performs the impedance translation at the design frequency. The common design
formulas account only for a resistive source and load. Since the output devices have parasitic capacitance, after the
network is designed, the series value of L4 may be adjusted to account for this capacitance. A value of L4 ≈ 0% to
25% higher than the calculated value is sometimes required to bring the network to peak efficiency.
12
1
V4
Tran Generators = PULSE
8
V6
14
2
V8
Tran Generators = PULSE
4
10
L3
5nH
10
2
C1
.02uF
20 7
L2
.15nH
12 20
R1
.15
15 3
L5
.15nH
14 15
R3
.15
7
1
V3
3
2
V7
3
3
4
1
V2
12
V9
IV4
10
10
IV8
2
22 2
14
14
1
V5
1
1
2
2
5
V1
163
8
R5
50
1 6
R2
.03
6 13
L4
96n
13 8
C2
.1u
8
17
C3
319p
8 8
17
R4
.01
4
5
L1
10nH
IV1
4
4
3 3
22
1
1
4
4
*
WR5
*
WV1
88
1
1
*
WR2
*
WR4
5 5
22
44
11 11
2
L6
10nH
1111
7
777
3
3
7
7
1
3
2
X1
ARF460AB
4
7
1
X2
ARF460AB
RF Output Network
Resonant and
Matching Network
High Frequency
Loop
Low Frequency Loop
Gate Driver
Figure 1. N - N Channel Half Bridge
Figure 1 illustrates an N-N Channel Half Bridge circuit Topology. The circuit contains two current loops. A low
frequency loop is highlighted in yellow. The High Frequency Loop is highlighted in red. These loops are illustrated
with near-minimum stray inductance. Great care should be taken to achieve inductance values near the illustrated
values of Figure 1. If we allow L2 to reach 100nH or greater, performance will be severely degraded. The
inductance of the Inner Loop, L3, is an extremely Critical Stray Component. Values greater than a few nH can cause
stability problems and excessive harmonics.
Table 1
Circuit Performance
Vsupply +197V
Pout 2036W
Pin 2351W
PLoss 315W
Eff 86.6%
Pulse Gate Drive PW=27ns
Ths
45°C
Tj X2
100°C
TjX1
100°C
Drain Z=3Ω Out Z=50Ω
Application Note 1816
May 2011
www.microsemi.com
2/3
Given the preceding Half-Bridge Design discussion, we have evaluated several ARF devices in this topology using a
Spice circuit simulation based or our new device models, and correlated the performances with previous bench work
by the author. Circuit Parameters have been adjusted for the Highest Efficiency and Highest Power Output while
limiting the MOSFETs junction temperature to a maximum of ≈ 100°C and limiting the Drain to Voltage to V
DS
Maximum. We have set the heat sink temperature at 45°C.
Figure 2 shows the resulting output of the simulations from 2MHz to 40MHz for each of the devices that were
selected for this Application Note.
Figure 2. RF Output Power vs. Frequency
In Figure 2 we see that the device with the dashed line plot seems very different than the other four devices. The
ARF300-ARF301 Half Bridge is limited in power by the characteristics of the P Channel device, the ARF301. The
proper drain load for the ARF301 is about 8Ω, for the ARF300 it is about 3Ω. The ARF300 is the device with the
lowest R
DS(on)
the ARF461 is the highest at 2Ω. The advantage of the ARF301 is the simplified drive circuit in Half
Bridge configurations, see Note 1.