Application Training Guide
Mounting Techniques
Most mounting questions concern the TO-220AB and TO-247AC package styles used to house diodes, MOSFETs, and IGBTs, and the three methods used to make electrical connections to the die within these packages: wire bonding, soldering, and compression mounting. Each method has its own advantages and disadvantages.
Figure 23. Wire Bonding
Wire bonding, shown in Figure 23, uses a small diameter (typically <=20 mils) wire that is ultrasonically bonded (melted) at each connection point. Advantages: wire bonding is quick and easy, and can be completely automated. Disadvantages: increased voltage drop due to the small wires, low fusing current, expensive equipment required.
Solder mounting shown in Figure 24 below is typically used in smaller diodes (< 10A), mostly the familar axial-leaded diode.
Figure 24. Solder Mounting
Some smaller IR Schottky diodes (SMB and SMC) also use this technique. Advantages: both the voltage drop, and fusing current are improved. For example, the 30BQ015 Schottky diode is rated at 3 amps and 15 volts, but the surge rating is 650 amps because the die is soldered directly to the leadframe. Disadvantage: both sides of the die must be solderable.
Figure 25. Compression Bonding
Compression bonding (Figure 25 above) is used in devices where power cycling capability is required, typically high power diodes and SCRs. Advantage: in these applications, compression bonding makes a much better connection because there is no fixed (soldered) interface to fatigue, and the fusing current is very high. Disadvantage: compression bonding is more expensive, and requires physically rugged die. All IR hockey puks are compression bonded. Some stud mounted diodes and SCRs, and some diode and SCR modules are also compression bonded.
Topical Applications
International Rectifier manufactures components used in the efficient control of energy. High efficiency is a major requirement because of today's rising energy costs and the need for battery-powered systems to be able to run extended periods of time.
In Figure 26, the line power or raw energy source may be an electric utility, automobile alternator (ac), battery (dc), or a power supply. IR components take this raw power and condition it into more useful energy. Some examples are switching off unneeded circuits in portable electronics, variable ac to control motors, and variable dc to control motors, electronic lamp ballasts, etc. All IR components fit into the basic power conversion function sub-blocks:
Figure 26. The Basic Power Conversion Functions
These blocks are similar to the USDA's Basic Food Groups wherein each meal should include one item from each group. Similarly, designs will often need one or more items from each group. The following sections discuss each block in detail, the associated IR product, and how to use them.
Input Rectification
Input rectification is the first stage in most electronic devices using ac power. The design uses four diodes/rectifiers arranged in a bridge configuration for single-phase inputs, or six diodes arranged in a bridge for three-phase inputs. Standard recovery diodes are specified since the speed of the diode is not important. Standard recovery diodes have excellent forward voltage drop, and have lower relative costs than other families of diodes.
Figure 27. Bridge Rectifier
A bridge rectifier (both single- and three-phase shown in Fig. 27 above) converts the ac waveform on the left to the dc level on the right. IR sells both single- and three-phase bridges in plastic isolated-base modules. Diode bridges can also comprise discrete diodes, or doubler diode modules (two diodes in series). The choice depends on the desired mounting method, and the current requirement.
In the above example, the dc voltage will equal the peak of the ac input voltage due to the capacitor on the output of the bridge rectifier. Since ac voltage is measured in volts-rms, peak voltage is equal to 1.414 times the rms voltage. Select the rectifiers based on this peak voltage, allowing extra margin for high line conditions.
SCRs may be used to limit current, or output voltage. In some cases, designers require SCRs in their input bridges. Phase control SCRs are used in smart bridge configurations, so called because they can be used to limit inrush current.
A problem with passive rectification is that current is only drawn from the line when the line voltage exceeds the capacitor voltage. This results in a current waveform shown on the left in the above diagram. The mismatch between the current and voltage waveforms is called power factor, and results in inefficient utilization of the power source. Additionally, some government regulations require new designs to meet a specific power factor.
Typical Applications
A boost converter (shown below) is the most common circuit used to improve the power factor. As shown on the left, the voltage and current waveforms are very similar. This results in near-perfect power factoring.
Figure 28. Boost Converter
Figure 29. DC Chopper
DC Chopper
The dc output voltage is in the 400 to 500 volt range, and calls for a 450 to 600 volt rating on both the power switch and the diode. IR has focused on this application with its low gate charge HEXFET® power MOSFETs and HEXFRED® ultrafast diodes. This configuration is one most commonly used in power electronics circuits. Depending on the application, a transformer may be used to provide voltage isolation or to change voltage levels according to the turns ratio between the primary and secondary sides. This single switch, single ended configuration is also known as a forward converter in the power supply world. In power supply applications, the inductance and the freewheeling diode are on the secondary side of the transformer.
Much emphasis is placed on the selection of the switch (either a MOSFET or IGBT), but the selection of the diode is equally important. The diode characteristics affect the operation of the switch itself. For low voltage applications, a Schottky diode is ideal. For higher voltage applications, IR's HEXFRED is an excellent choice.
Figure 30. Half Bridge
Half Bridge
The half-bridge is a higher power version of the previous circuit. It is the workhorse of the power electronics industry, finding use in power supplies, motor controls, and lighting ballasts. In a power supply, the inductor is actually a transformer. In a motor drive, the inductor is the motor windings. In a lighting ballast, the inductor is in series with the lamp, which is in parallel with the lower capacitor.
The greatest design problem with the half-bridge configuration is driving the upper switch, which can be either an IGBT or a HEXFET. To properly drive a MOS-gated transistor, the gate voltage must be greater than the emitter/source voltage by approximately 15V. When the emitter/source terminal is connected to a fixed voltage reference (i.e., ground in the case of the bottom switch), this is a simple task. However, the emitter/source of the high-side switch swings between ground (when the lower switch is conducting), and nearly the positive rail, which requires the gate voltage to be above the positive rail. This is typically a problem, since the positive rail is usually the highest voltage available in the system.
Several methods are used to solve the problem of driving the upper switch (for a list, see AN978A), all of which are relatively complex, and each has drawbacks. IR produces a range of devices to solve this problem: the IR2100 Series Control IC drivers. They use a technique called bootstrapping to generate the gate drive signal for the upper switch. IR manufactures a line of these devices that focus on the electronic ballast market, the IR215x. They include both control circuitry and gate drive circuitry--all on one chip.
In even higher power circuits, the two capacitors on the right side of the circuit may be replaced with a half-bridge identical to the one on the left. This is called a full bridge. In addition to higher power applications, full bridges are also used in reversible dc motor drives. The configuration allows voltage to be applied to the load in both directions.
Typical Applications
Figure 31. Three-Phase Bridge
The three-phase bridge can be thought of as three half-bridges. Three-phase outputs are mainly used for motor drives or ac inverters. The IR2130 Control IC has been designed specifically for this application.
Figure 32. Push-Pull Configuration
The push-pull configuration is used in power supply and low power UPS systems.
Figure 33. Two Transistor Forward Converter
The two transistor forward converter topology is commonly used in power supply and switched reluctance (SR) motor designs. Its advantage is that the voltage requirement of the switch and freewheeling diode is half that of the one transistor forward topology, often used in off-line power supplies requiring the greater capabilitiesof 800 volt HEXFET® power MOSFETs. The increased benefits and lower cost outweigh the complexity of the design. Typical applications include low power converters with high switching frequencies.
Figure 34. High Side dc Switch
Placing the switch on the high side offers protection against the most common short circuit, a short to the chassis (ground). If this occurs, the load is connected to ground on both ends, so the ground is unenergized. However, if the switch and load were reversed, a short to ground would energize the load.
IR manufactures the lR62xx series of high side intelligent switches. They offer overcurrent, overtemperature, and ESD protection. And because these devices accept ground-referenced logic-level signals as control, the problem of driving a high-side switch is addressed.
In some applications, high-side intelligent switches can be used to replace electromagnetic relays. However, applications that require voltage isolation must use IR microelectronic relays. These solid state devices cost more than equivalent mechanical devices, but feature superior reliability. Additionally, microelectronic relays are available to control ac loads. This line has found application in telecommunications, instrumentation, and process control.
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