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Application Note

DC+ Bus Power-Supply Solution Using Bootstrap Charge
Pump Technique

Eason Tian

Precision ADCABSTRACT

DC+ (DC bus positive side) overcurrent protection is widely used in industrial systems. Compared with thetraditional discrete solution, the isolated comparator (AMC23C11) can achieve better performance. This article
introduces a cost-effective approach for generating the power supply for the isolated comparator sitting on theDC+ side. This approach uses a bootstrap charge pump technique to generate a stable power supply on DC+.

Table of Contents1 Background.............................................................................................................................................................................22 Overcurrent Protection on DC+.............................................................................................................................................33 DC+ Power Supply Solution.................................................................................................................................................. 4
3.1 Selection of Charge Pump Capacitor.................................................................................................................................53.2 Simulation in PSpice® for TI...............................................................................................................................................5
3.3 Hardware Test.................................................................................................................................................................... 7
3.4 Low-Cost Power Supply Solution.......................................................................................................................................74 Power-On Short-Circuit Risk and Solution...........................................................................................................................85 Reference................................................................................................................................................................................ 9

List of Figures
Figure 2-1. DC+ Overcurrent Protection...................................................................................................................................... 3
Figure 3-1. DC+ Power Supply Solution...................................................................................................................................... 4Figure 3-2. Simulation Model.......................................................................................................................................................5
Figure 3-3. 1 kHz, 20% Duty Cycle Power Supply...................................................................................................................... 6
Figure 3-4. Increase Capacitor Value From 1 μF to 30 μF.......................................................................................................... 6
Figure 3-5. Low-Cost Power Supply Solution.............................................................................................................................. 7
Figure 4-1. Output Behavior for Long Settling Times of the Reference Voltage (AMC23C11).................................................... 8
Figure 4-2. Add DC– Short-Circuit Protection..............................................................................................................................8

List of Tables
Table 3-1. 1-kHz Switching Frequency........................................................................................................................................ 7
Table 3-2. 20-kHz Switching Frequency...................................................................................................................................... 7

Trademarks
PSpice® is a registered trademark of Cadence Design Systems, Inc..
All trademarks are the property of their respective owners.

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1Background

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1 Background
Fault-detection mechanisms are a necessity in high-power industrial systems such as motor drives and solar
inverters, as well as automotive systems including electric vehicle (EV) chargers, traction inverters, onboard
chargers and DC/DC converters. DC bus-based overcurrent protection is widely used in electric motor drives.
The traditional implementation of overcurrent (OC) fault detection is discrete with a combination of non-isolated
multichannel comparators and either optocouplers or digital isolators. To meet the growing needs of fault
detection, TI is introducing a new family of basic and reinforced isolated comparators to the TI isolation
portfolio. The primary use case is ultra-fast overcurrent, overvoltage, over temperature detection in high-voltage
industrial and automotive HEV/EV systems. Its smaller PCB area is particularly suitable for applications withminiaturization and high-power density needs. Compared with traditional solutions, it has significant advantagesin CMTI, response time, threshold accuracy, hysteresis, and latching function.
The power supply on DC+ for overcurrent protection can be generating by either adding a transformer or adding
an additional secondary winding to an existing transformer. However, technical challenges such as transformer
size limitations or the proximity of these transformers to the actual OC implementation will practically limit such atransformer-based implementation.

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Overcurrent Protection on DC+2 Overcurrent Protection on DC+Figure 2-1 shows how the OC protection based on the DC+ bus can protect three short-circuit situations: ashoot-through fault of IGBT (blue), DC+ ground fault (red), and phase-to-phase short fault (yellow).

OC protection

MFigure 2-1. DC+ Overcurrent Protection
Adding an isolated comparator such as a single threshold comparator AMC23C11 for OC detection ensuresappropriate detection and protection of power circuits from these three conditions. The dual-threshold version(AMC23C14) can achieve both short circuit and overload protection.
As there is no need for a power supply on DC+ other than for an isolated comparator, a simple, low-cost, andlow BOM implementation is critical.

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DC+ Power Supply Solution

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3 DC+ Power Supply Solution
Figure 3-1 shows the implementation of a DC+ power supply.

Figure 3-1. DC+ Power Supply Solution
The DC+ power supply solution consists of two parts. The first part is a bootstrap circuit consisting of ahigh-voltage diode D1, a capacitor C7 and current-limiting resistor R6. C7 is charged by the low-side gate driver
power supply. The second part is a charge pump consisting of a high-voltage diode D2, a capacitor C2 andcurrent-limiting resistor R4. When the half bridge starts to operate there are three operating states as follows:•••

When the low-side IGBT turns on, C7 is charged by the power supply of the low-side gate driver and D2 isturned off.
When the high-side IGBT turns on, D2 conducts, C7 charges the isolated comparator and capacitor C2.
When the low-side IGBT turns on again, C7 is charged by the low-side power supply and C2 continues tosupply power to the isolated comparator.

In summary, this charge-pump based bootstrap circuit provides power to the isolated comparators with the helpof energy stored in capacitors C7 and C2 during the IGBT switch phases.

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DC+ Power Supply Solution

3.1 Selection of Charge Pump Capacitor
The selection of the two capacitors, C2 and C7, is important to minimize the ripple on the generated powersupply. The following is an example to calculate the capacitor value under extreme conditions. The good news
is that the integrated low-dropout (LDO) regulator on the high-side of the AMC23C11 eases the pre-regulation
requirements of this power supply. As an example, this design is capable of taking a ripple voltage of 3 V, andother parameters are as follows:••••

Half-bridge circuit switching frequency f = 1 kHzDuty cycle D = 20%
Current required for isolated comparator I = 3.3 mA (max)Low-side driver supply U_low side = 15 V

Switching frequencies in motor drive systems are typically 1 kHz to 20 kHz. Lower switching frequencies result
in larger ripple because of longer discharge times. Increasing the capacitance reduces the ripple but increases
the charging time. Assume the switching frequency is 1 kHz and 20% duty cycle in the extreme case. This casemeans that the isolated comparator is powered only by the capacitor C2 of the charge pump for 80% of a PWM
cycle, the minimum capacitance required for C2 under this condition is:
Q = I × t = ΔU × C(1)×t
C = IΔU= 3.3 mA3×V0.8 ms = 0.88 μF

(2)

V×2
IRboot = 15 V −100.7= 1.36 A
Ω(3)In this design, C2 takes a capacitance of 1 μF and the bootstrap capacitor C7 is also 1 μF. R4 and R6 limit thehigh currents that may occur during initial power-up. Typical values for this resistor are 5 Ω to 10 Ω. A larger
resistance increases the time constant of the RC circuit and prolong the time to reach the minimum supply.
When R4 is taken as 10 Ω, the maximum current at initial power-up is 1.36 A:

The two diodes D1 and D2 should be able to withstand the voltage of DC bus. For a motor drive system witha 380 VAC input, the diodes need to have a withstand voltage value ≥ 1200 V, and must have fast reverse
recovery characteristics to minimize the recovery charge and thereby ensure the stability of the supply circuit.
The circuit can theoretically generate a supply voltage of 13.6 V, which is 15 V minus the voltage drop of the twodiodes (0.7 V).

3.2 Simulation in PSpice® for TI

Figure 3-2. Simulation Model
The DC bus in the simulation model is 600 V, a 2.7-mA current source is used to simulate the isolatedcomparator, the square wave source V5 simulates the switch of the half bridge. The low-side supply is 15 V.
Both capacitors, C2 and C7, are 1 μF and the current-limiting resistors R3 and R5 are 5 Ω.
The supply voltage of the isolated comparator (V_DC+) and the half-bridge center point voltage (V_half bridge)
are measured at 1-kHz switching frequency and 20% duty cycle as Figure 3-3 shows.
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Figure 3-3. 1 kHz, 20% Duty Cycle Power Supply
In Figure 3-3, the power supply is stabilized between 11.25 V and 8.92 V after a few PWM cycles, that is, there
is a ripple of 2.33 V. Since the half-bridge circuit turns on the upper IGBT first in the first cycle, the isolatedcomparator is not powered until one cycle later, that is, the system is not protected from power-up short-circuit in
the first cycle. An optimized solution to minimize risk is provided in Section 4.
Keeping the same switching frequency and duty cycle, increase the C2 and C7 simultaneous capacitance value
from 1 μF to 30 μF gradually. This method reveals that the supply voltage gradually increases and gets closer tothe ideal value (13.6 V), and the power supply ripple also decreases.

Figure 3-4. Increase Capacitor Value From 1 μF to 30 μF
In Figure 3-4, V_MAX is the maximum voltage value that can be achieved with a stable supply. Therefore,
controlling the capacitance of the two capacitors above 5 μF to obtain a more stable supply voltage isrecommended. But a capacitance value that is too large will also increase the charging time.

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DC+ Power Supply Solution

3.3 Hardware Test
The actual circuit is built according to the circuit diagram in Figure 3-1, where the capacitor C2 = C7 = 1 μF, R4 =10 Ω, R6 = 5 Ω, the low-side driver supply is 15 V, and the DC bus voltage is 10 V. The supply voltage and ripple
are tested at different duty cycles for 1 kHz and 20 kHz.
Table 3-1. 1-kHz Switching Frequency
Duty Cycle

V_MAX

Ripple20%

11.4 V3.44 V50%

11.9 V

2.96 V80%

12.2 V

2.16 V

Table 3-2. 20-kHz Switching Frequency
Duty Cycle

V_MAX

Ripple20%

12.9 V352 mV80%12.9 V

104 mV

Table 3-1 and Table 3-2 show that lower switch frequency and lower duty cycle bring lower supply voltage(V_MAX) and larger ripple. A similar result is obtained for the 150-V DC bus voltage.

3.4 Low-Cost Power Supply Solution
The bootstrap circuit in Section 3.1 is similar to the power supply of the high-side gate driver. Through reuse, thispower supply can reduce the cost. As shown in Figure 3-5, VCC is the power pin of the high-side gate driver andthis solution removes a high-voltage diode (D1) and RC (C7, R6).
AMC23C11

R4VCC
D2High sidegate driver

C2OUT

VDD1

VDD2IN

LATCH

REF

OUT

GND1

GND2DC+VEE

Figure 3-5. Low-Cost Power Supply Solution
The solution shown in Figure 3-5 increases a load for the power supply of the gate driver, so it is necessary
to pay attention to the impact of the gate driver. Increasing the current-limit resistance (R4) can reduce theimpact. This solution still has the limitation because the isolated comparator power supply is dependent on thehalf-bridge circuit operation.

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Power-On Short-Circuit Risk and Solution

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4 Power-On Short-Circuit Risk and Solution
Since the bootstrap circuit and charge pump need to wait for one cycle of half-bridge operation before they
power the isolated comparator. After VDD1 has powered up to greater than 3 V, the capacitor on the reference
pin must charge to the threshold voltage before the isolated comparator is functional. Figure 4-1 shows that
output of isolated comparator is valid when reference voltage is within 1% of the set value.
VDD1

VDD2

ON
<1%OUT1 not valid
if VIN is in this range
during power-up

VREF
VREF settling time
OUT1

lowOUT2

lowOutput not valid

Output valid

Output validtHS,STA + tHS,BLKFigure 4-1. Output Behavior for Long Settling Times of the Reference Voltage (AMC23C11)
Therefore, the boot strap charge pump solution is not able to provide OC protection during the initial start-upof the motor drive. This risk can be reduced by adding DC– OC protection and pre-turn on the three low-sideIGBTs.

OC protection

MOC protection

Figure 4-2. Add DC– Short-Circuit Protection
The added DC– protection can share power with the low-side driver, so there is always protection on the DC–.
The DC– protection protects against both shoot-through fault (blue) and phase-to-phase short fault (yellow).
However, the DC+ ground fault (red) cannot be protected directly through the DC–. Turn on the three lower
IGBTs before the IGBT module starts working to detect whether there is a short to ground, and also to charge
the bootstrap capacitor.

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Reference

5 Reference
1. Texas Instruments, TIDA-010036 One-phase shunt electricity meter reference design using standalone
ADCs Reference Design
2. Texas Instruments, AMC23C11 Fast Response, Reinforced Isolated Comparator With Adjustable Thresholdand Latch Function Data Sheet
3. Texas Instruments, Using Isolated Comparators for Fault Detection in Electric Motor Drives Analog DesignJournal

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