Power Diodes and Rectifiers (Redrestor)

Power Diodes are semiconductor pn-connections that can transmit large currents at high voltage values for use in rectifier circuits.

In previous lessons, we have seen that a semiconductor signal diode will transmit the current from the anode to the cathode (forward direction) in only one direction, but not in the opposite direction, acting as a one-way valve.

A widely used application of this feature and diodes in general is the conversion of an alternative voltage (AC) to a continuous voltage (DC).In other words, it's pointing.

Small signal diodes can only be used as rectifiers in low-power, low-current (less than 1 amp) rectifiers or applications, but in cases of larger forward pre-voltage currents or higher reverse pre-charge blocking voltages, the PN connection of a small signal diode eventually overheats and melts, instead larger, more robust Power Diodes are used.

Semiconductor diodes, simply known as Power Diodes, have a much larger PN connection area than their smaller signal diode cousin, resulting in high forward current capacity of up to several hundred amps (KA) and upward blockade voltage (several thousand volts (KV)).

Since the power diode has a large PN connection, it is not suitable for high frequency applications above 1MHz, but special and expensive high frequency, high current diodes are available.For high frequency rectifier applications, Schottky Diodes are often used due to short recovery times and low voltage drop in forward pre-voltage situations.

Power diodes allow uncontrolled power to be directed and are used in applications such as battery charging and DC power supplies and AC rectifiers and inverters.Due to their high current and voltage properties, they can also be used as free rotating diodes and blocking networks.

Power diodes are designed to have advanced "ON" resistance of the fractions of an Ohm, while reverse blocking resistance is in the mega-Ohm range.Some of the larger power diodes are designed to be "studded" to coolers that reduce their thermal resistance to 0.1 to 1 o C/Watt.

If alternative voltage is applied to a power diode, the diode will transmit the passing current during the positive half-cycle and the diode will not block the flow of the current during the negative half-cycle.Then transmission from the power diode occurs only during a positive half-cycle, and therefore, as shown, there is a one-way pass, which is DC.

Power Diode Rectifier

power diodes

Power diodes can be used separately as above or connected to produce various rectifier circuits such as "Half Wave", "Full Wave" or "Bridge Rectifier".Any type of rectifier circuit can be classified as uncontrolled, semi-controlled or fully controlled; Here it uses only power diodes for an uncontrolled rectifier, uses thyristors (SCRs) for fully controlled rectifiers, and the semi-controlled rectifier is a mixture of both diodes and thyristors.

The most commonly used individual power diode for basic electronic applications is the general purpose 1N400x Series Glass Suppressed type rectifier diode with continuously forwarded current standard ratings of approximately 1 amp and reverse blockage voltage ratings from 50v to 1N4007 to 1000v for 1N4001. The 1N4007GP diode is the most popular for general purpose mains voltage correction.

Half Wave Rectifier

The Half Wave Rectifier is a circuit that converts Alternating Current (AC) input power to Direct Current (DC) output power.The input power supply can be a single-phase or multi-phase welding, the simplest of all rectifier circuits is that of the Half Wave Rectifier.

The power diode in the half-wave rectifier circuit passes only half of each full sine wave of the AC source to convert it to a DC source.Such circuits are called "half wave" rectifiers because they pass only half of the incoming AC power supply, as shown below.

power diodes

During each "positive" half cycle of the AC sinus wave, the diode is pressed forward, causing the current to flow through the diode, since the anode is positive compared to the cathode.

Since the DC load is resistant (resistance, R), the current flowing in the load resistance is therefore proportional to the voltage (Ohm Law) and therefore the voltage on the load resistance will be the same as the feed voltage Vs.(minus Vε ), that is, the "DC" voltage on the load is only for the first half cycleit is sinusoidal, so Vout = Etc.

During each "negative" half cycle of the AC sinusoidal input waveform, the diode is inverted because the anode is negative compared to the cathode.Therefore, the diode or circuit does not pass through the current.Then, in the negative half-cycle of the feed, the current does not flow in the load resistance as no voltage appears on it, so the Vout becomes 0 at that time.

The current on the DC side of the circuit flows in one direction and makes the circuit one way.When load resistance receives a positive half of the waveform from the diode, zero volts, positive half of the waveform, zero volt, etc., the value of this irregular voltage will be equal to the equivalent DC voltage of 0.318*Vmax. Input is 0.45*Vrms of sinusoidal waveform or input sinusoidal waveform.

Then the equivalent DC voltage throughout the load resistance, V DC is calculated as follows.

power diodes

Where V MAX is the maximum or peak voltage value of ac sinusoidal feed, and V RMS is the RMS (Root Average Square) value of the feed voltage.

Power Diodes Question Example 1

Calculate the VDC and IDC,which pass through 100Ω resistance connected to a 240 VRMS single-phase half-wave rectifier, and the average DC power consumed by the load.

power diodes

During the straightening process, the resulting DC voltage and current are both "ON" and "OFF" in each cycle.Since the voltage on the load resistance exists only in the positive half of the cycle (50% of the input waveform), this results in a low average DC value provided to the load.

The change in the directional output waveform between this "ON" and "OFF" state produces a waveform that contains large amounts of "ripples", an undesirable feature.The resulting DC fluctuation has a frequency equal to the AC feed frequency.

Very often, when pointing an alternative voltage, we want to produce a "constant" and continuous DC voltage without any voltage variation or fluctuation.One way to do this is to connect a large value Capacitor to the output voltage terminals in parallel with the load resistance, as shown below.This type of capacitor is commonly used in the "Warehouse" or Smoother/It is known as the Softener Capacitor.

Half Wave Rectifier with Smoothing/Softening Capacitor

power diodes

When straightening is used to provide the correct voltage (DC) power supply from an alternative (AC) source, the largerThe amount of surge voltage can be further reduced by using capacitors, but softening types have both cost and size limitations.

For a specific capacitor value, a larger load current (smaller load resistance),discharges the capacitor faster (RC Time Constant) and therefore increases the resulting fluctuation.Then, for a single-phase, half-wave rectifier circuit using a power diode, stand-aloneIt is not very practical to try and reduce the surge voltage by softening the capacitor.In this case, it will be more practical to use "Full Wave Straightening" instead.

In practice, the half-wave rectifier is the most commonly used straightening circuit in low-power applications due to its large disadvantages.The output amplitude is smaller than the input amplitude, there is no output during the negative half-cycle, so half the power is wasted, and the output becomes the pulsed DC, which results in excessive fluctuation.

To overcome these disadvantages, we will review the Full Wave Rectifier in the next tutorial.