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A Smart Solid-State Fuse Cures,http://www.5idzw.com
The MAX668 then tries to compensate by raising its output voltage, possibly resulting in an overvoltage condition. As an antidote, make sure the feedback resistors (the minimum DC load on the main output voltage) are small enough to discharge Vout slightly faster than the discharge of C3 by Q2's emitter current. Whether Q1 conducts or not, the following inequality lets you size C3:
Many components and interconnects affect the MAX668 feedback path shown in Figures 3 and 4, and a fault in these components can produce an overvoltage Vout that destroys the load. For added safety, a zener diode (not shown) connected from C1 to the MAX668 FB pin (with its anode connected to FB) can provide an overriding local feedback loop that clamps the output at (Vz + VFB). To prevent a large overvoltage, set Vz equal to the maximum regulated Vout minus the maximum VFB.
If the system must control multiple loads individually while the boost converter remains on, you can replace the MAX810 with a MAX812 (in a 4-pin SOT143 package). The MAX812's fourth pin is designed for manual-reset applications, but it can force a disconnect between the local load and main boosted output by acting as a logic-level signal that overrides each smart solid-state fuse. This approach lets you control each load on the main boost supply independently.
Last but not least, this smart solid-state fusing technique, which auto-resets without power cycling and requires no replacement or field troubleshooting, need not be limited to boost-converter outputs. It can replace a fuse on the DC-power bus of virtually any system, regardless of voltage. (Bus voltages higher than 60V can require non-logic-level FETS and level shifters for the MAX810 output.) Using just two precision resistors to set an appropriate external bias for the higher voltages (Figure 6), you can set the solid-state fuse to be triggered by a programmed sag in the bus supply voltage.
Figure 6. Extending the load-disconnect idea to non-boost-converter circuits forms a solid-state fuse that is applicable to any DC-power bus.
Suppose, for example, that -48 volts are to be protected against overcurrent. We interrupt the rail side instead of the ground side, because the voltage source is negative and we use an n-channel FET plus a MAX809T reset circuit, whose reset-output polarity is opposite to that of the MAX810. The supply voltage can range down to -36V under normal operation (Figure 7).
Figure 7. This solid-state fuse protects a negative DC-power bus.
Design equations are as follows:
The MAX809 quiescent current is about 100µA maximum over temperature, and the current through Rh and RL should be about 100 times' higher to minimize the effect of quiescent current on trip voltage: 36/(Rh+RL) = 10 mA, therefore
Rh should have a power rating of 0.5W. Because the voltage across RL exceeds the MAX809's maximum input-voltage rating when Vsupply goes above its minimum limit, place a 5V ±5% zener diode across RL as shown.
，A Smart Solid-State Fuse Cures
The MAX668 then tries to compensate by raising its output voltage, possibly resulting in an overvoltage condition. As an antidote, make sure the feedback resistors (the minimum DC load on the main output voltage) are small enough to discharge Vout slightly faster than the discharge of C3 by Q2's emitter current. Whether Q1 conducts or not, the following inequality lets you size C3:
C4 can be small compared to C3 if no charging pulses are skipped during normal PWM operation; but the more pulses are skipped, the larger C4 has to be. When boosting action resumes after pulse skipping, while Q2 is held off, C4 should be large enough to charge C3 before C1 becomes charged fully.
(Vout-Vbe)/(R1 × C3) < Vout/[(Ra+Rb)×(C1+C2)].
Many components and interconnects affect the MAX668 feedback path shown in Figures 3 and 4, and a fault in these components can produce an overvoltage Vout that destroys the load. For added safety, a zener diode (not shown) connected from C1 to the MAX668 FB pin (with its anode connected to FB) can provide an overriding local feedback loop that clamps the output at (Vz + VFB). To prevent a large overvoltage, set Vz equal to the maximum regulated Vout minus the maximum VFB.
If the system must control multiple loads individually while the boost converter remains on, you can replace the MAX810 with a MAX812 (in a 4-pin SOT143 package). The MAX812's fourth pin is designed for manual-reset applications, but it can force a disconnect between the local load and main boosted output by acting as a logic-level signal that overrides each smart solid-state fuse. This approach lets you control each load on the main boost supply independently.
Last but not least, this smart solid-state fusing technique, which auto-resets without power cycling and requires no replacement or field troubleshooting, need not be limited to boost-converter outputs. It can replace a fuse on the DC-power bus of virtually any system, regardless of voltage. (Bus voltages higher than 60V can require non-logic-level FETS and level shifters for the MAX810 output.) Using just two precision resistors to set an appropriate external bias for the higher voltages (Figure 6), you can set the solid-state fuse to be triggered by a programmed sag in the bus supply voltage.
Figure 6. Extending the load-disconnect idea to non-boost-converter circuits forms a solid-state fuse that is applicable to any DC-power bus.
Suppose, for example, that -48 volts are to be protected against overcurrent. We interrupt the rail side instead of the ground side, because the voltage source is negative and we use an n-channel FET plus a MAX809T reset circuit, whose reset-output polarity is opposite to that of the MAX810. The supply voltage can range down to -36V under normal operation (Figure 7).
Figure 7. This solid-state fuse protects a negative DC-power bus.
Design equations are as follows:
The MAX809 quiescent current is about 100µA maximum over temperature, and the current through Rh and RL should be about 100 times' higher to minimize the effect of quiescent current on trip voltage: 36/(Rh+RL) = 10 mA, therefore
The MAX809 threshold is much lower than the supply-trip voltage, so RL is smaller than Rh, approximately by the ratio Vthreshold/(Vthreshold + Vsupply-trip) = 3/(36+3) = 0.077. Thus, MAX809 Iq flows through ~93.3% of (Rh+RL), causing a voltage-trip contribution of ~0.336V. Taking this fact into account, set the initial trip voltage for calculating Rh and RL at 36V - 0.336V = 35.664V. Using 1% resistors for Rh and RL, Vsupply-trip = 35.664V. This threshold occurs when the MAX809T threshold is at its minimum (3.15V, over the temperature range -40°C to + 85°C):
(Rh+RL) = 3600 Ohms.
Calculated values for RL and Rh are 323.81Ω and 3276.19Ω, respectively. The closest 1% values are 320 and 3280. Considering these resistor values and the 100µA Iq, the maximum supply-trip voltage becomes 36.09V, which surpasses 36V slightly. This result also occurs only for simultaneous worst-case values for all errors, which are rare scenarios in practice. For most applications, the foregoing design would be quite acceptable. The MAX809's nominal threshold voltage gives a nominal trip voltage of -34.65V.
35.664V[RL(0.99)/(RL(0.99)+Rh(1.01))] = 3.15V.
Rh should have a power rating of 0.5W. Because the voltage across RL exceeds the MAX809's maximum input-voltage rating when Vsupply goes above its minimum limit, place a 5V ±5% zener diode across RL as shown.
，A Smart Solid-State Fuse Cures