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Diodes in parallel?

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2022-11-14 10:31

Diodes are often used in work, but why can diodes only be connected in series? Today we will explore the symmetrical distribution of static cut-off voltage and dynamic cut-off voltage when the machine is connected in series. In static state, because the cut-off leakage current of each component in series has different manufacturing deviation, the component with the minimum leakage current bears the maximum voltage, even reaches the holding state. However, as long as the component has sufficient holding stability, it is unnecessary to use a voltage equalizing resistor in the circuit. It is generally necessary to add a parallel resistance only when the components with cut-off voltage greater than 1200V are connected in series. Assuming that the cut-off leakage current does not change with the voltage, and the resistance error is ignored, for the series circuit of n diodes with a given cut-off voltage VR, we can obtain a simplified formula for calculating the resistance: the above Vm is the maximum value of the voltage in the series circuit, and △ Ir is the maximum deviation of the diode leakage current, provided that the operating temperature is the maximum value. We can make a safe assumption: in the above formula, Irm is given by the manufacturer. Using the above estimate, the current in the resistor is about six times the diode leakage current. Experience shows that when the current flowing through the resistance is about three times the diode leakage current under the maximum cut-off voltage, the resistance value is sufficient. But even under this condition, considerable losses will still occur in the resistance. In principle, the dynamic voltage distribution is different from the static voltage distribution. If the carrier hour of one diode pn junction is faster than that of the other, it will also withstand the voltage earlier. If the capacitance deviation is ignored, then when n diodes with a given cut-off voltage value Vr are connected in series, we can use a simplified method to calculate the parallel capacitance: the above △ QRR is the maximum deviation of diode storage capacity. We can make a fully safe assumption: the condition is that all diodes are from the same manufacturing batch number, and △ QRR is given by the semiconductor manufacturer. In addition to the stored power when the freewheeling diode is turned off, the power stored in the capacitor also needs to be replaced by the IGBT being turned on. According to the above design formula, we find that the total storage capacity may be twice that of a single diode. Generally speaking, the series current of freewheeling diode is rare, because of the loss source of the following accessories: 1. n-fold diffusion voltage of pn junction; 2. Loss in parallel resistance; 3. Additional storage power that needs to be replaced by IGBT; 4. Increase of components caused by RC circuit. Therefore, when the diode with high cut-off voltage can be used, the series connection scheme is generally not adopted. The only exception is that when the application circuit requires a very short switching time and a very low storage capacity, these two points are exactly what the grounded diode has. Of course, the on state loss of the system will also be greatly increased. Parallel connection does not require additional RC buffer circuit. It is important that the deviation of on state voltage should be as small as possible during parallel connection. An important parameter to judge whether a diode is suitable for parallel connection is the dependence of its on state voltage on temperature. If the on state voltage decreases with the increase of temperature, it has a negative temperature coefficient. This is an advantage for loss. If the on state voltage increases with temperature, the temperature coefficient is positive. In a typical parallel application, this is an advantage because the hotter diode will withstand lower current, resulting in system stability. Because there is always a certain manufacturing deviation for diodes, a large negative temperature coefficient (>2mV/K) may cause the risk of temperature rise imbalance when diodes are connected in parallel. The diode in parallel will generate thermal coupling: 1. The module with multiple chips in parallel will pass through the substrate; 2. When multiple modules are connected in parallel to a heat sink, the heat sink is generally used for weak negative temperature coefficient. This kind of thermal coupling is sufficient to avoid the temperature imbalance of the diode with the lowest on state voltage. However, for diodes with negative temperature coefficient>2mM/K, we suggest derating, that is, the total rated current should be less than the sum of the rated current of each diode.
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