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| 3-phase diode bridge plus thyristor PGH series Power Module PGH series power module includes 3-phase diode bridge and inrush current limiting thyristor in a package. This series are widely applied to rectification circuit in popular 3-phase inverters. This paper shows how to use PGH series, and also covers information on 3-phase rectification circuit, driver circuit, and selection of heatsink. In addition, it provides designers, who are not very familiar with thyristor, with its basic application information. E-36 E-15 34mm 75mm 41.5mm 97.5mm E-43 62mm 108mm PGH series PGH series Packages List of PGH series Part Number PGH308 PGH3016AM PGH508AM PGH5016AM PGH758AM PGH7516AM PGH1008AM PGH10016AM PGH1508AM PGH15016AM PGH2008AM PGH20016AM IT(AV), IF(AV) (A) 30 30 50 50 75 75 100 100 150 150 200 200 VDRM ,VRRM (V) 800 1600 800 1600 800 1600 800 1600 800 1600 800 1600 Case Outline E-15 E-36 E-36 E-36 E-36 E-36 E-36 E-36 E-43 E-43 E-43 E-43 1 S. Hashizume Dec., 2007 Rev.1.0 Id AVG IdAVG IdAVG e RMS e RMS e RMS Id AVG Rectification circuit e RMS e RMS e RMS eRMS Output voltage Average current ( ( Resistive load) Inductive load) IdAVG 1.57 IdAVG 3.14 IdAVG 0.5 IdAVG 0.785 IdAVG 1.57 IdAVG 0.5 IdAVG 0.707 IdAVG IdAVG 0.5 IdAVG 0.785 IdAVG 1.57 IdAVG 0.5 IdAVG 0.707 IdAVG IdAVG 1.41 eRMS 1.57 0.333 IdAVG 0.579 IdAVG 1.05 IdAVG 0.333 IdAVG 0.578 IdAVG IdAVG 2.45 eRMS 1.05 1.73 Each diode Each diode RMS current Peak current Average current RMS current Peak current Peak reverse voltage to Diode DC output voltage Peak /Average AC input voltage Line voltage/Phase Voltage AC input voltage RMS / Average 1.41 eRMS 3.14 2.82 eRMS 1.57 2 2.22 1.11 1.11 0.428 Table 1 Constants of rectification circuits Diode current 200V RMS 200V RMS 28.2 200V RMS Diode voltage 490V An example of diode current and voltage in three phase full-wave rectification circuit 3 follows. For AC200V line : VDRM and VRRM : 800V For AC400V line : VDRM and VRRM : 1,600V Between AC line and bridge rectifier, use appropriate AC line filter. It reduces the noise entering into the equipment not so as to cause any undesired behaviors. Furthermore, it suppresses conducted emission from the equipment. As are expected, external stresses on diode and thyristor, such as surge voltage and current, can be decreased by such filter. Thyristor 1, What's thyristor Thyristor is considered as a diode and a switch connected in serial. Anode Cathode Gate Thyristor Same as bipolar transistor, thyristor is driven by current. With respect to bipolar transistor, when base current is applied, collector current of hFE times base current flows. In contrast, thyristor is switched on by gate current that is higher than a specific value (gate trigger current). The following figure illustrates these relationships. You will see that collector current of bipolar transistor flows during the whole period when base current flows, but thyristor keeps anode current flowing even after the gate current is cut off. So, you need not supply thyristor with continuous gate current i TDK 3-phase line filter and internal diagram Thyristor (SCR) Anode E i E vT v iG iG Gate Cathode Bipolar transistor (NPN) Collector E iC E iC hFExiB vCE(sat) vCE iB iB Base Emitter 4 during all of the on-period. At present, major switching devices, such as MOSFET and IGBT, are driven by voltage, but thyristor is currentdriven device. Please keep in mind this fact when you design gate firing circuit for thyristor. IT e Gate trigger current e Gate current Gate current turns on Thyristor 2, Behaviors of Thyristor as a switch -- Holding current, and Latching current 2-1, Holding current Once thyristor turns on, the on-state is maintained as far as anode current is larger than a certain value. In other words, thyristor turns off when anode current decreases to a certain value. The "certain current" is the holding current, and that of PGH308 (30A 800V) is 70mA typical at 25. (Refer to individual datasheet.) Now, let's see the influence of holding current in an actual circuit. Anode current decreases. IT Thyristor turns off when anode current becomes below holding current. minimum anode current which can maintain onstate is the latching current. For example, typical latching current of PGH308 (30A 800V) is 90mA 25). Pulse gate current Time Anode current Latching current Thyristor turns off because of slow rise of anode current. Holding current Thyristor turns off. Time time ON OFF Latching current If thyristor cannot be turned on or on-state may not be able to maintain, increase gate pulse width or try multiple gate pulses. Both holding current and latching current are temperature-dependent, and they become larger at low temperature. Compared with at 25, they are about twice larger at -40. Holding current Supposing that pulse trigger current is applied only once. Thyristor is turned on, however, if the load is resistive and anode-cathode voltage goes to zero, anode current altogether decreases to below holding current. After that, positive voltage would be applied to anode, however, thyristor maintains off-state so far as gate current would be applied again. 2-2. Latching current Assume that, due to slow rise of anode current, the current doesn't reach a certain level before gate current is terminated, thyristor turns off. It follows that, after removal of gate current, the 5 3, Gate drive 3-1 How to achieve sure turn-on 3-1-1 Temperature dependence of gate characteristics In datasheet of PGH308, you will find a graph of gate characteristics like this. x2 Factor of pulse gate current x1 0 2s 5s 10s 20s 50s Pulse width Typical pulse trigger current DC. Assuming that the minimum operating temperature is -20 and pulse width is 5s, the estimated peak trigger current is 300mA (150mAx 2). Accordingly, combination of dependence in temperature and dependence in pulse width will give you how large is the required gate current to trigger. This graph shows required gate current and voltage to trigger all the PGH308 at -40, at 25 and 125. For example, we know that DC current of 100mA can turn on every PGH308 at 25, and accompanied gate voltage is less than 2.5V. Based on this graph, let us find out how large is the gate trigger current at a certain temperature, which comes from the lowest operating temperature of the equipment in which the thyristor will be installed. Trigger currents at -40 , 25, and 125 are plotted on the graph like below, and we can estimate that trigger current at -20 is around 150mA. 200mA Power:Less than 5W 100mA All triggered at-40 Gate current : less than 2A 0 -50 0 50 100 150 3-2 Ratings of gate current, voltage, and power Rating is the limit where stress on device may spoil its reliability significantly or cause catastrophic damage. As shown on the graph below, the three ratings - peak gate current, voltage, and power (gate current times gate voltage) - are defined. In addition, average gate power is also limited. For detailed information, refer to individual datasheet. Gate voltage : less than 10V 3-1-2 Pulse width dependence of gate trigger current In case that pulse gate current is applied, and pulse width is shorter than 20s, required gate current to turn on thyristor is large compared with DC. Furthermore, a remarkable increase in gate trigger current is needed when the pulse duration falls below 10 s specifically. For example, pulse trigger current of 5s width is twice larger than Trigger gate current Junction temperature Gate ratings To turns on thyristor firmly, gate current and gate voltage tend to become high. Be careful in average power for DC triggering, and in peak power for pulse triggering. 3-3 To avoid trigger by noise (To avoid malfunction) The maximum gate voltage not to trigger is 0.25V Tj1252/3VDRM. This implies that more than 0.25V between gate and cathode may possibly turn on the thyristor. Temperature dependence of gate current 6 In order to avoid unintended turn-on by noise (malfunction), such measures are expected to be effective. *Connect cathode of trigger signal to the terminal ex- Terminals for trigger clusive for trigger. *Gate serial diode Noise as high as diode forward voltage (approximately 0.7 V) is cancelled. However, the drive signal is cut by the voltage, and, if necessary, it should be compensated.. *Gate parallel diode The diode may prevent an excessive gate reverse voltage. *Gate parallel capacitor 0.010.1F nal resistance). Accordingly, considering minimum operating temperature and width of triggering pulse, we can design gate driver that can turnon every device, where drive current, voltage, and power are all well within the corresponding ratings. As shown in the figure below, at first, plot opencircuit power-supply voltage of the gate trigger circuit on the voltage axis (vertical axis), and plot short-circuit current at the current axis (horizontal axis). Then, link these two points by straight line. This gate load line should exceed area that all devices can be triggered, and should also satisfy all the ratings - gate current, gate voltage, and gate power. In this example, short-circuit current is 0.5A, and open voltage is 8V. Therefore, we know that the current-limiting resistance is 16 . Total16 8V Design example of gate load line The load line is a classical way of thinking. At present, we can easily realize constant-voltage or constant-current drivers. IGBT and MOSFET are driven by voltage, however, thyristor is driven by current. Consequently, when designing gate driver for thyristor, apply constant-current basis design. Incidentally, reverse power loss of thyristor increases significantly in case of applying DC gate current while reverse voltage is applied to anode to cathode. Because reverse voltage isn't applied to PGH in standard applications, this fact is not meaningful. However, remember that it's an important nature of thyristor. , Thermal design (Choice of heatsink) Including PGH, base plate of power module is generally made of copper. However, unless combined with heatsink, temperature rise is so 30A Measures to avoid gate malfunction 3-4 Gate load line A gate load line is used to specify the powersupply voltage to gate trigger circuit, and currentlimiting resistance (including power supply inter- Gate to cathode voltage :less than 10V Power : less than 5W All triggered considering operating temperature and pulse width Gate current: less than 2A Gate load line PGH508AM 7 and 10ms. Here, the I is RMS current. Assuming that 1 pulse surge on-state current is 600A, I2t can be calculated as follows. 60022x0.011,800A2s This figure is useful when thyristor is protected by (cutting) fuse. There is a similar regulation in fuse, too, so we can choose a matched pair where thyristor doesn't fail but fuse is broken. Critical rate of rise of turn-on current di/dt defines how large is the destructive limit below 2ms. After gate current is applied, it takes about 100 s before all the area of thyristor turns into on-state. In other words, if pulse width of current is very short, partial conduction occurs. As a result, small area owes the power, and power density in the area also becomes very high. It is the di/dt that, for such reason, prescribes the rating against sharp rising current pulse. These three current rating are represented on common time axis as follows. I2t di/dt 50Hz ITSM 200A at 2 s, after turn-on (after gate current begins to flow). The initial turned-on area depends on gate drive current. The faster and the larger on-gate is, the larger initial turn-on area is. For that reason, faster and larger gate current, such as iG=200mA and , diG/dt=0.2A/s, is specified as standard condition for di/dt for a thyristor that has maximum trigger gate current of 50mA at 25 C. When high di/dt is anticipated, additional reactor in the anode current loop is effective to suppress di/dt. Additionally, enough large and sharp ongate current within gate ratings, is also valid to improve di/dt capability of thyristor itself. 5-2 Critical rate of rise of off-state voltage dv/ dt As explained, thyristor is normally turned on by gate current. However, it may be also turned on by high dv/dt of anode voltage. It is the critical rate of rise of off-state voltage dv/dt, which prescribes the limit of rising. Displacement current into inner capacitance of thyristor chip has similar effect to gate current. The dv/dt is a typical cause of thyristor malfunctions. Thyristor chips which have dv/dt capability of 100V/s or more have internal resistance that can bypass displacement current. Countermeasures against malfunction by dv/dt include application of thyristor that has higher dv/ dt capability, addition of RCD to gate circuit same as for noise, and controlling dv/dt itself by CR snubber. 2ms 10ms At present, we don't worry whether major power switching devices, such as MOSFET or IGBT, would withstand starting-up current or not even if how fast it is. This is because very small unit-cells are accumulated in one chip, and their high frequency characteristics are remarkably excellent compared with thyristor. By contrast, general thyristor is made of single thyristor unit. Therefore, on-region begins from neighborhood area of gate, and it spreads to the whole chip with time. Gate Cathode On-region spreads with time Excessive dv/dt is applied Thyristor turns on. On-region spread of Thyristor chip If critical rate-of-rise of on-state current di/dt is 100A/s, for example, thyristor may fail when anode current reaches more than 100A at 1 s, 10 |
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