AND9285/D Low Saturation Transistor for Gate Drive Application 1. Overview www.onsemi.com IGBTs or MOSFETs that control high current of more than 100A are used as power discrete device. Usually these devices have large size because they operate high current. Consequently, large gate capacitance is required for powering on these devices, so it is necessary to consider gate driving in order to achieve fast turn-on and off. Specifically, a gate drive circuit (current buffer circuit) is required as a driving device to instantaneously charge and discharge the large gate capacitance (Cies, Ciss). Therefore, low-saturation Bipolar Transistor with low operating voltage (approx.1V) and current operating ability is used for the gate drive circuit of the IGBT or MOSFET. APPLICATION NOTE 1-1 is a single circuit or bridge circuit’s low-side circuit. When driving ability of the Drive IC is not sufficient, fast turn-on/off can be enabled by adding a PNP/NPN Bipolar transistor with totem-pole connected between the gate of the IGBT (MOSFET). 1-2 is an isolated input circuit where input is composed of insulated circuit. When the current driving ability of the Optocoupler is not sufficient, as is the case with 11, the driving ability can be improved by adding a Bipolar Transistor. 1-3 is a non-isolated circuit: fast switching operation is enabled by adding a PNP/NPN BipTR circuit to the high side of the half-bridge driver IC and such kind of BipTR circuit also to the low side (bridge circuit). 2. Drive Circuit Configuration Below shows several methods of actual gate drive circuit. Typical example is shown as Table.1. PNP and NPN complementary device is used. Rg1 DriveIC Gate Driver IC Rg2 1-1.Single Circuit TND525SS Opto Coupler 1-3.Non-Isolated (Half Bridge) circuit 1-2.Isolated Input circuit © Semiconductor Components Industries, LLC, 2015 August 2015- Rev. 0 1 Publication Order Number: AND9285/D AND9285/D 3. How to select the gate drive transistor (1) 3-1) About VCEO Because it is operated below the IGBT (or MOSFET)’s gate voltage (VGES or VGSS), this VCEO becomes an indicator. In case of IGBT, gate operating voltage is usually VGE=15~16V, so we recommend the gate drive TR’s voltage to be VCEO=30V. However, as shown in 1-2, in case the drawing side is negative, the voltage of gate drive TR becomes VBB+|VEE|, when considering the margin, we recommend VCEO=50V. VBB Q1 Therefore, in consideration of the TR’s speedup, the higher the IGBT’s current specification and Cies are, the more necessary to make the gate drive TR’s current capacitance (Icp) larger. VBB=15V Rg Cies I IG Fig.2 Equivalent circuit of RC Rg As shown in Fig.2: when considering the current flowing the gate TR (IGp), taking the power supply VBB=15V, Rg=10Ω, Gate charge current = IGp of the gate drive TR, then, IGp VBB÷Rg = 15÷10=1.5 [A] So, a gate drive TR of Icp>1.5A is required. Q2 VEE Fig.1 Gate Drive circuit 3-2) Current specification The specifications shall meet the following requirements. 1) Gate drive current: IGp IGp is determined by the difference between the applied voltage (+VBB) and VEE (typically minus) and the external resistance (Rg). IGp (VBB – VEE) / Rg 2) Specification of IGBT(or MOSFET) and IGp Typically, Cies becomes larger as the current specification of IGBT becomes higher. Switching speed (tr or tf) is expressed as in Fig.2 when simply considering the circuit as RC circuit. When you want to keep tr constant, the larger the Cies becomes, the smaller the Rg becomes. Gate voltage rise time (tr) at this time is calculated as follows: R: equivalent to external gate resistance Rg C: Cies of the IGBT Assuming VBB is constant, C’s charge voltage’s final value (=VBB), then, tr is equivalent to the time from the point of (VC=VBB x 10%) to the point of (VC=VBB x 90%). The calculation of tr period in RC circuit (t10% t90%) is as below: Tr=2.2CR(calculated from charge time of C). For example, When Cies=5000pF, Rg=10Ω, Gate voltage rise time’s theoretical value is calculated as tr=110ns. www.onsemi.com 2 AND9285/D 3-3) How to select the gate drive transistor (2) Vcc=400V As shown in Fig.3: in case of changing the drive ability at ON side and OFF side of the IGBT. Vout VBB=15V Q1 IG ON Rg Rg1=22Ω PG D1 Rg2=4.7Ω Q2 Fig.4 Test circuit without Gate driver IG OFF VEE=-5V Vcc=400V Gate drive Trs NPN+PNP In case of emitter-common (Fig.3), IGON and IGOFF are sometimes set as separate path, considering Rg as R1 and R2 in parallel, then, IGp = (VBBVEE)/(Rg1×Rg2÷(Rg1+Rg2)) IGp = (15+5) / (22×4.7÷(22+4.7)) = 5.17A For this circuit, devices of Icp > 5.2A are recommended. If the device is s single item, current specification of Q1and Q2 can be different. With respect to Icp: Icp expresses the maximum absolute rating of pulse current, which is under the condition of Ton(ON time) < 1μs, pulse duty < 1% and Tc=25C. In fact, because Ta > 25C, you should take this into account when selecting device. The same is true for the case when the operation’s repetition frequency is high, sometimes you should take the temperature rise into account. 3-4) Comparison When actually operating IGBT, we compared the characteristics of the 2 cases: without gate driver and with gate driver. Fig.4 is the case without gate driver; Fig.5 is the case with gate driver. We compared IGBT’s output side (collector-emitter side) switching loss: Eon (loss at the time when Ic begins to flow) + Eoff (loss at the time of Ic cutoff). The smaller one [(the smaller value of (Eon + Eoff)] is considered to be able to reduce the loss of the circuit. L=200μH Vout Rin=56Ω Rg1=22Ω Rg2=5.6Ω PG Fig.5 Test circuit with Gate driver In case of without gate driver, the value is small, but (Eon + Eoff) did not decrease even when making Rg lower than 5.6Ω. However, when adding the gate driver circuit, the value of (Eon + Eoff) further decreased by 5%, which contributes greatly to the overall circuit efficiency. (Fig.6) 1600 With Gate-driver or Non Comparison of the Eon + off 1500 Vcc=400V L=200 H Ic=20A 1400 Eon+Eoff[ J] Fig.3 Gate Drive circuit (Rg1 Rg2) 1300 1200 Non (5.6Ω) 1100 Gate driver (22Ω/5.6Ω) 1000 900 800 Non (5.6Ω) Gate driver (22Ω/5.6Ω) Fig.6 Comparison of (Eon + Eoff) www.onsemi.com 3 AND9285/D 4. Lineup We provide the following lineup of gate drive transistors according to input capacitance and voltage. Table.2 and Table.3 are NPN and PNP single products separately. Table 2. TR NPN for gate drive Table 3. TR PNP for gate drive VCEO IC MCPH3 CPH3 PCP (V) (A) 2 MCH3245 CPH3245 2SC5994 50 3 CPH3223 2SC5964 VCEO IC MCPH3 CPH3 PCP (V) (A) 2 MCH3145 CPH3145 2SA2153 50 3 CPH3123 2SA2125 Complex type (NPN and PNP housed in 1 package) lineup is shown in Table 4. Standard value in the table is an indicator of the gate capacitance of the IGBT (or MOSFET) that can deliver the performance of the gate drive transistor. For example, when IGBT(or MOSFET) input capacitance Cies=5000pF, CPH5506 can be recommended. Table.4: TR complex type for gate drive Package Name Standerd value IGBT'S (MOSFET) Cies (Ciss) [pF] VCEO up to 3000 up to 10000 up to 15000 up to 40000 50V CPH5 ECH8 SOT-23-5 2.9×2.8×0.9 ICP*1(IC ) 3A(1A) CPH5518 5A(2A) CPH5520 6A(3A) CPH5524 *2( ECH8502 30A 5A) E common B1 E1/E2 B2 E separate C C C C E B Electrical connection C1 C2 *1 PW 10s, Duty cycle 1% *2 PW 1s, Duty cycle 1% www.onsemi.com 4 E B AND9285/D ON Semiconductor and the ON logo are registered trademarks of Semiconductor Components Industries, LLC (SCILLC) or its subsidiaries in the United States and/or other countries. SCILLC owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property. A listing of SCILLC’s product/patent coverage may be accessed at www.onsemi.com/site/pdf/Patent-Marking.pdf . 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