CN0196

Circuit Functions and Advantages



This circuit is an H-bridge composed of high-power switching MOSFETs, controlled by low-voltage logic signals, as shown in Figure 1. This circuit provides a convenient interface between low-level logic signals and high-power bridges. Low-cost N-channel power MOSFETs are used on both the high and low sides of the H-bridge. This circuit also provides isolation between the control side and the power side. This circuit can be used in applications such as motor control, power conversion with embedded control interfaces, lighting, audio amplifiers, and uninterruptible power supplies (UPS).
Modern microprocessors and microcontrollers are generally low-power and operate from low supply voltages. The 2.5 V CMOS logic outputs source and sink currents in the μA to mA range. In order to drive a 12 V switching, 4 A peak current H-bridge, the interface and level translation components must be carefully selected, especially when low jitter is required.
The ADG787 is a low-voltage CMOS device with two independently selectable single-pole double-throw (SPDT) switches. With a 5 V DC supply, the effective high-level input logic voltage can be as low as 2 V. Therefore, the ADG787 is able to provide the conversion of the 2.5 V control signal to 5 V logic levels required to drive the ADuM7234 half-bridge driver.
The ADuM7234 is an isolated half-bridge gate driver that uses Analog Devices' iCoupler® technology to provide independent and isolated high-side and low-side outputs, allowing the exclusive use of N-channel MOSFETs in the H-bridge. There are many benefits to using N-channel MOSFET: the on-resistance of N-channel MOSFET is usually only 1/3 of that of P-channel MOSFET, and the maximum current is higher; switching speed is faster and power consumption is reduced; rise time and fall time is symmetrical.
The ADuM7234's 4 A peak drive current ensures that the power MOSFETs can be turned on and off at high speeds, minimizing power dissipation in the H-bridge stage. In this circuit, the maximum drive current of the H-bridge can be as high as 85 A, which is limited by the maximum allowable MOSFET current.
The ADuC7061 is a low-power, ARM7-based precision analog microcontroller with an integrated pulse width modulation (PWM) controller. Its output can be used to drive the H bridge after appropriate level conversion and conditioning.











Circuit Description



2.5 V PWM Control Signal Level Translation to 5 V
EVAL-ADuC7061MKZ provides a 2.5 V logic level PWM signal, but the ADuM7234 has a minimum logic high input threshold of 3.5 V on a 5 V supply. Because of this incompatibility, an ADG787 switch is used as an intermediate level converter. The minimum input logic high control voltage of the ADG787 is 2 V, which is compatible with the 2.5 V logic of the ADuC7061. The output of the ADG787 switches between 0 V and 5 V, which is sufficient to drive the 3.5 V threshold ADuM7234 input. The evaluation board provides two jumpers for easy configuration to control the polarity of the PWM signal.
Introduction to H Bridge The
H bridge shown in Figure 1 has 4 switching elements (Q1, Q2, Q3, Q4). These switches are turned on in pairs, the upper left side (Q1) and the lower right side (Q4) are a pair, and the lower left side (Q3) and the upper right side (Q2) are a pair. Note that switches on the same side of the bridge will not be on at the same time. The switch can be implemented using a MOSFET or IGBT (Insulated Gate Bipolar Transistor), using a pulse width modulation (PWM) signal or other control signal from the controller to turn the switch on and off, thereby changing the polarity of the load voltage.
The sources of the low-side MOSFETs (Q3, Q4) are connected to ground, so their gate drive signals are also referenced to ground. The source voltage of the high-side MOSFETs (Q1, Q2) switches as the MOSFET pairs are turned on and off, so this gate drive signal should be referenced or "bootstrapped" to this floating voltage.
The ADuM7234's gate drive signals allow true galvanic isolation between the input and each output. Each output operates up to ±350 VPEAK relative to the input, allowing low-side switching to negative voltages. As a result, the ADuM7234 can reliably control the switching characteristics of various MOSFET configurations over a wide range of positive or negative switching voltages. To ensure safety and simplify testing, a 12 V DC power supply was selected as the power supply for this design. The
gate
driver power supplies for the high-side and low-side of the bootstrap gate drive circuit are different. The low-side gate drive voltage is referenced to ground, so the drive is powered directly from the DC source. However, the high side is left floating, so a bootstrap drive circuit is required, which works as described below.
Looking at the left side of the H-bridge circuit shown in Figure 1, the bootstrap drive circuit is implemented using capacitor C1, resistors R1 and R3, and diode D1. After power-on, PWM does not occur immediately, all MOSFETs are in a high-impedance state until all DC voltages have completed establishing. During this time, capacitor C1 is charged by the DC source through paths R1, D1, C1 and R3. The charged capacitor C1 provides the high-side gate drive voltage. The time constant of C1 charging is τ = (R1 + R3) C1
When the MOSFET is switched under the control of the PWM signal, the low-side switch Q3 is turned on and the high-side switch Q1 is turned off. The high-end GNDA is pulled down to ground, and capacitor C1 is charged. When Q1 is on, Q3 is off and GNDA is pulled up to the DC supply voltage. Diode D1 is reverse biased, and the C1 voltage drives the VDDA voltage of the ADuM7234 to approximately 24 V. Therefore, capacitor C1 maintains approximately 12 V between the VDDA and GNDA pins of the ADuM7234. In this way, the gate drive voltage of the high-side MOSFET Q1 is always referenced to the floating source voltage of Q1.
Voltage spike on the source of the high-side MOSFET
When Q1 and Q4 are turned on, the load current flows from Q1 through the load to Q4 and ground. When Q1 and Q4 are disconnected, current still flows in the same direction, through freewheeling diodes D6 and D7, creating a negative voltage spike at the source of Q1. This may harm some gate drivers using other topologies, but has no effect on the ADuM7234, which supports low-side switching to negative voltages.
Bootstrap Capacitor (C1, C2)
The bootstrap capacitor charges every time the low-side driver turns on, but it only discharges when the high-side switch turns on. Therefore, the first parameter to consider when selecting the bootstrap capacitor value is the maximum allowable voltage drop when the high-side switch is turned on and the capacitor is used as the high-side DC supply for the gate driver ADuM7234. When the high-side switch is on, the ADuM7234 draws a typical dc supply current of 22 mA. Assuming a high-side switch on-time of 10 ms (50 Hz, 50% duty cycle), using the formula C = I × ΔT/ΔV, if the allowable voltage drop ΔV = 1 V, I = 22 mA, ΔT = 10 ms , the capacitance should be greater than 220 μF. This design chooses a capacitance value of 330 μF. When the circuit is powered off, resistor R5 will discharge the bootstrap capacitor; when the circuit switches, R5 has no effect. When
the bootstrap current-limiting resistors (R1, R2)
charge the bootstrap capacitor, the series resistor R1 plays a current-limiting role. If R1 is too high, the DC quiescent current from the ADuM7234 high-side drive power supply will cause an excessive voltage drop on R1, and the ADuM7234 may undervoltage lockout. The maximum dc supply current of the ADuM7234 is IMAX = 30 mA. If the voltage drop across R1 caused by this current is limited to VDROP = 1 V, then R1 should be less than VDROP/IMAX, or 33 Ω. Therefore, this design chooses a 10 Ω resistor as the bootstrap resistor.
Bootstrap starting resistor (R3, R4)
Resistor R3 starts the bootstrap circuit. After powering on, the DC voltage does not build up immediately and the MOSFET is in the off state. Under these conditions, C1 is charged through the paths R1, R3, D1, and VS. The process is described in the following formula:
where vC(t) is the capacitor voltage, VS( is the power supply voltage, VD( is the diode voltage drop, and τ is the time Constant, τ = (R1 + R3) C1. The circuit values ​​are: R1 = 10 Ω vC1 = 330 μF, VD = 0.5 V, VS = 12 V. From the above equation, the capacitor charges to its final value when R3 = 470 Ω. 67% takes a time constant (158 ms). The larger the resistor value, the longer it takes to charge the capacitor. However, when the high-side MOSFET Q1 is turned on, there will be 12 V across resistor R1, so if the resistor If the value is too low, it may dissipate considerable power. For R3 = 470 Ω, the power dissipation of this resistor is 306 mW at 12 V.
Overvoltage protection of the bootstrap capacitors (Z1, Z2)
is as mentioned above for inductive loads. , when the high-side MOSFET is turned off, current flows through the freewheeling diode. Due to the resonance between the inductor and the parasitic capacitance, the charging energy of the bootstrap capacitor may be higher than the energy dissipated by the ADuM7234, and the voltage on the capacitor may rise to an overvoltage condition. . The 13 V Zener diode clamps the voltage across the capacitor to avoid overvoltage conditions.
Gate drive resistor (R7, R8, R9, R10)
Gate resistor (R7, R8, R9, R10) as required. Switching time tSW. Select. Switching time is the time required to charge Cgd, Cgs and the switching MOSFET to the required charge Qgd and Qgs.






Description Gate drive current Ig:

where VDD is the supply voltage and RDRV is the gate driver ADuM7234. The equivalent
resistance of the ADuM7234 gate driver is calculated by:
According to the ADuM7234 data sheet, for VDDA = 15 V and output short-circuit pulse current IOA(SC) = 4 A, RDRV is approximately 4 Ω according to Equation 3.
According to the FDP5800 MOSFET data sheet, Qgd = 18 nC, Qgs = 23 nC. , Vgs(th) = 1 V.
If the required switching time tSW is 100 ns, solving for Rg from Equation 2 shows that Rg is approximately 22 Ω. The actual design selects a 15 Ω resistor to provide some margin for
power rail filtering . Undervoltage Protection
Due to the high peak load current, the DC supply voltage (VDD) must be properly filtered to prevent the ADuM7234 from entering undervoltage lockout and to prevent possible damage to the power supply. The selected filter consists of four 4700s in parallel. A µF, 25 V capacitor is connected in series with a 22 µH power inductor, as shown in Figure 2. The capacitor is rated for a maximum rms ripple current of 3.68 A at 100 kHz. Since the 4 capacitors are connected in parallel, the maximum allowed rms ripple is 14.72 A. Therefore, IPEAK = 2√2 × IRMS = 41.63 A.
The filtered +12 V voltage also drives the circuit shown in Figure 1.
When the supply voltage falls below 10 V, the circuit shown in Figure 2 disables the input of the ADuM7234, thereby preventing the ADuM7234 from undervoltage lockout. Applying a logic high signal to the DISABLE pin of the ADuM7234 disables this circuit.
The ADCMP350 open-drain active low comparator is used to monitor the DC supply voltage. The ratio of the resistor divider (R12, R13) is chosen so that when the supply voltage is 10.5 V, the voltage divider output is 0.6 V, which is equal to the comparator's on-chip reference voltage of 0.6 V. When the supply voltage drops below 10.5 V, the output of the comparator goes high. Because there is galvanic isolation between the input and output of the ADuM7234, the DISABLE signal at the output must be transmitted to the input through the isolator.
The ADuM3100 is
a digital isolator based on iCoupler technology. The ADuM3100 is compatible with 3.3 V and 5 V operating voltages. The filtered 12 V supply voltage drives the linear regulator ADP1720, which provides 5 V (+5V_1) to the right isolated terminal of the ADuM3100, as shown in Figure 2.
Load and PWM Signal
If an inductor is used as the load, the current flowing through the inductor will change linearly when a constant voltage is applied. The voltage U is 12 V. If the MOSFET voltage drop caused by the on-resistance is ignored, the following equation holds:
For a 50 kHz, 8% duty cycle PWM signal, when using a 4 μH Coilcraft power inductor (SER2014-402) as the load, the load The current waveform is shown in Figure 3. Use a current probe to measure the inductor current.
For a 12 V supply voltage and a 4 μH inductor, Equation 4 predicts a slope of 3 A/μs. The measured slope is 2.8 A/μs, and the reason for the decrease in slope is the voltage drop caused by the MOSFET on-resistance.
Note that a small amount of ringing oscillation will appear on the waveform for a short period of time after the current is disconnected. The reason is the resonance between the inductive load and the parasitic capacitance of the freewheeling diode and MOSFET.
Care must be taken that the inductor current in the circuit must not exceed its rated maximum. If it is exceeded, the inductor will be saturated and the current will increase rapidly, possibly damaging the circuit and power supply. The Coilcraft SER2014-402 inductive load used in this circuit has a saturation current rating of 25 A.