Mosfet In Series And Parallel

Posted : admin On 1/3/2022

HVDC design engineer by trade here. You don't want to use MOSFETs for the simple reason of cost (chip area) per A of current and since MLC concept was published, everyone is switching at low speed. Paralleling both IGBTs and MOSFETs becomes a challange at kA levels due to parasitics and fault cases. Series connecting is a major challange. Don't do it unless you absolutley have to and be prepared for. Have you ever noticed that the brightness of LEDs change differently when you connect them in different ways? This tutorial explains the differences. In this article you will learn how to calculate LEDs in series and parallel using a simple formula and configure your own personalized LED displays, now you don't just have. Several visitors to this website have tried to connect power MOSFET transistors in parallel in order to switch a higher power load. Here I'll explore that issue and why problems may arise. 1 N-channel MOSFETs connected in parallel. Fig.1 illustrates 4 n-channel MOSFETs connected in parallel. At issue is Rg the gate bleeder resistor. You may recall that in Capacitance, we introduced the equivalent capacitance of capacitors connected in series and parallel.

The intrepid power systems designer should know all about MOSFETs and their particular electrical peculiarities, but working with arrays of MOSFETs can be another beast. One arrangement you might see in a power conversion system is to place multiple power MOSFETs in parallel. This shares the load among multiple MOSFETs with the goal of reducing the burden on the individual transistors in your system.

Unfortunately, MOSFETs (and nonlinear components in general) do not simply divide up the current among themselves in the same way as, say, a group of resistors in parallel. Just like in a single MOSFET, the heat now becomes a consideration as it determines thresholding behavior in MOSFETs (again, this applies to any real nonlinear circuit). To see how these components interact with each other in this arrangement, we need to look at the parasitics that exist within a MOSFET chip and between power MOSFETs in parallel so that you can prevent components from destroying themselves.

Working with Parallel MOSFETs

Like any other component, be it linear or nonlinear, multiples of the same component or circuit network can be connected in parallel. This is also true for power MOSFETs, BJTs, or other groups of components in your schematics. For 3-terminal devices like MOSFETs, where power must be supplied at two terminals, the configuration involved may not be so intuitive. The schematic below shows an example from a power converter where four MOSFETs are hooked up in parallel on the converter’s output side.

Parallel

Note that there is a small resistor connected to the gate on each MOSFET (I’ll explain why in a moment). There is also a single gate pulse from a synchronous driver at the VG_PWM port, which is used to switch each MOSFET simultaneously. In other words, these MOSFETs are not driven in a cascaded manner; they are driven such that they all switch on and allow current to flow at the same instant.

The advantages of hooking up MOSFETs in this way is that they can each be used to provide lower current to a load. In other words, the total current is split evenly among each MOSFET, assuming they have the same ON-state resistance. This allows each power MOSFET to provide high current while still having high current margin, which then reduces the amount of heat they generate.

Two points aren’t included in the typical analysis of power MOSFETs in parallel: parasitics in the MOSFET. Parasitics already create bandwidth limiting, filtering, or resonance effects in real components. However, when we have multiple power MOSFETs in parallel being driven with a high-frequency PWM signal, their parasitics can interact with each other and increase the possibility of an unwanted oscillation during switching. This would then appear as a glitch on the system output and can lead to excessive heating in the victim MOSFET.

Simulating Power MOSFETs in Parallel

When you have multiple power MOSFETs in parallel, and you want to simulate how parasitic oscillations might arise, you can build a simple circuit with a gate driver for your particular MOSFETs. Make sure you’ve attached the appropriate simulation model to your component, where the model includes stray capacitance between the various pins in the component. An example circuit with a load on the source side is shown below.

I’ve used a VPULSE source from the Simulation Sources.IntLib library to model a PWM driver. The diode D1 is a 1N914 diode arranged in a gate driver circuit for an NMOS transistor. From here, you simply need to perform transient analysis to examine the current and power delivered to the load by the MOSFETs.

Note that there are a few quantities that are of interest in this simulation:

  • PWM rise time: this determines the bandwidth of the PWM signal and should be matched to the specs for your MOSFET
  • PWM frequency: a PWM signal with higher frequency will see lower impedance from the parasitic capacitance, which injects more power into the parasitic feedback loop, possibly driving the system into resonance.
  • Gate voltage: Because a MOSFET’s response depends on the magnitude of the gate voltage, so will any parasitic oscillation that arises when the PWM signal switches the parallel array.

You can easily spot the effects of parasitic inductance and parasitic capacitance in a transient simulation. The example below shows results for the pair of MOSFETs above when the parasitic capacitance and inductance are included in the simulation model. Note the large glitches that are clearly seen in the time-domain response as the PWM signal switches.

Mosfet In Series And Parallel Equations

Damping Unwanted Oscillations and Temperature Rise

As was mentioned earlier, these unwanted oscillations can arise in different MOSFETs in the array if there is a temperature imbalance. In other words, the condition for resonance in one MOSFET can be different than in another MOSFET. If one MOSFET experiences strong oscillations before the other MOSFETs for a given gate voltage, then the component can destroy itself. Therefore, it’s best to keep these components at the same temperature if they are connected in series. This can be done with a large heatsink or a plane layer below the components in your PCB layout.

The other way to modify the conditions for resonance is to place a gate resistor in the driving circuit (see above, where a small 5 Ohm resistor is included). MOSFETs in half-bridge LLC resonant converters may have a very large resistor connecting the sources and gate to provide high damping between these two ports. You can experiment with these resistor values to examine how they affect damping in the parallel circuit.

Analog simulation is a central part of circuit design, including for power MOSFETs in parallel. The circuit design and PCB layout tools in Altium Designer® give you a complete set of features to help you create your circuits, simulate signal behavior, and create your PCB layout. Once you’ve qualified your schematic design, you can share your design data on the Altium 365® platform, giving you an easy way to work with your design team and manage your design data.

We have only scratched the surface of what is possible to do with Altium Designer on Altium 365. You can check the product page for a more in-depth feature description or one of the On-Demand Webinars.

In this article you will learn how to calculate LEDs in series and parallel using a simple formula and configure your own personalized LED displays, now you don't just have to wonder how to wire led lights? but actually can do it, know the details here.

These lights are known not only for their dazzling color effects, but also because of their durability and least power consumption.

Moreover LEDs can be wired in groups to form large alphanumeric displays which may be used as indicators or advertisements.

Young electronic hobbyists and enthusiasts are often confused and wonder how to calculate LED and its resistor in a circuit, since they find it difficult to optimize voltage and current through the group of LEDs, required to maintain an optimum brightness.

Why we need to Calculate LEDs

Designing LED displays may be fun, but very often we are just left thinking how to wire led lights? Learn through a formula how simple it is to design your own LED displays.

We already know that a LED requires a particular forward voltage (FV) to get lit. For example a red LED will need a FV of 1.2 V, a green Led will require 1.6 V and for a yellow LED it is around 2 V.

The modern LEDs are all specified with approximately 3.3V forward voltage irrespective of their colors.

But since the given supply voltage to an LED would be mostly higher than its forward voltage value, adding a current limier resistor with LED becomes imperative.

Therefore let's learn how a current limiter resistor may be calculated for a selected LED or a series of LEDs

Calculating Current Limiter Resistor

The value of this resistor may be calculated through the below given formula:

R = (supply voltage VS – LED forward voltage VF) / LED current I

Here R is the resistor in question in Ohms

Vs is the supply voltgae input to the LED

VF is the LED forward which is actually the minimum supply voltage required by an LED for illuminating with optimal brightness.

When a series LED connection is in question, you will just need to replace the 'LED forward voltage' with 'total forward voltage' in the formula, by multiplying FV of each LED by the total number of LEDs in the series. Suppose there are 3 LEDs in series then this value becomes 3 x 3.3 = 9.9

LED Current or I refers to the current rating of the LED, it may anywhere from 20 mA to 350 mA depending on the specification of the selected LED. This must be converted to amps in the formula, so 20 mA becomes 0.02 A, 350 mA becomes 0.35 A and so on.

How to Connect the LEDs?

To understand this let's read the following discussion:

Let's assume you want to design a LED display having 90 LEDs in it, with a 12V supply to power this 90 LED display.

To optimally match and configure the 90 LED with the 12V supply, you will need to connect the LEDs in series and parallel appropriately.

For this calculation we will need 3 parameters to be considered which are as follows:

  1. Total number of LEDs which is 90 in our example
  2. Forward voltage of the LEDs, here we consider it to be 3V for sake of easy calculation, normally this would be 3.3V
  3. The supply input, which is 12V for the present example

First and formost we have to consider the series connection parameter, and check how many LEDs can be accomodated within the give supply voltage

We do this by dividing the supply voltage by 3 volts.

The answer will be obviously = 4. This gives us the number of LED that could be accommodated within the 12V supply.

However the above condition may not be advisable because that would confine the optimal brightness to a strict 12V supply and in case the supply reduced to some lower value would cause lower illumination on the LED.

Therefore to ensure a lower margin of at least 2V it would be advisable to remove one LED count from the calculation and make it 3.

So 3 LEDs in series for a 12V supply looks good enough and this would ensure that even if the supply was reduced to upto 10V, still the LEDs would be able to light up quite brightly.

Now we would want to know how many such 3 LED strings could be made from our total 90 LEDs in hand? Therefore, dividing the total number of LEDs (90) by 3, we get an answer that's equal to 30. Meaning you would need to solder 30 numbers of LED series strings or chains, each string having 3 LEDs in the series. That's, pretty easy going right?

Once you finish assembling mentioned the 30nos of LED strings, you would naturally find that each string having its own positive and a negative free ends.

Next, connect the calculated value of resistors as discussed in the previous section to any one of the free ends of each series, you can connect the resistor at the positive end of the string or the negative end, the position doesn't matter because the resistor just needs to be in line with the series, you may even include some wher in between the LED series.Using the earler we find the resistor for each LED string to be:

R = (supply voltage VS – LED forward voltage VF) / LED current

= 12 - (3 x 3) / 0.02 = 150 ohms

Let's assume we connect this resistor to each of the negative ends of LED strings.

  • After this, you can begin joining the common positive ends of the LEDs together, and the negative ends or the resistor ends of each series together.
  • Finally apply 12 volts supply to these common ends as per the correct polarity. You will instantly find the whole design glowing up brightly with an uniform intensity.
  • You may align and organize these LED strings as per the design of the display.

LEDs with an Odd Count

A situation may arise when your LED display contains LEDs in odd numbers.

For example, suppose in the above case instead of 90 if the display would have consisted 101 LEDs, then considering 12V as the supply, it becomes a rather awkward task to divide 101 with 3.

So we find the nearest value which is directly divisible with 3 which is 90. Dividing 99 with 3 gives us 33. Therefore the calculation for these 33 LED strings would be as explained above but what about the remaining two LEDs? No worries, we can still make a string of these 2 LEDs and put it in parallel with the remaining 33 strings.

However to ensure that the 2 LED string consumed uniform current just like the remaining 3 LED strings, we calculate the series resistor accordingly.

In the formula we simply change the total forward voltage as show below:

Mosfets

R = (supply voltage VS – LED forward voltage VF) / LED current

= 12 - (2 x 3) / 0.02 = 300 ohms

This gives us the resistor value specifically for the 2 LED string.

Therefore we have 150 ohms for all the 3 LED strings, and 300 ohms for the 2 LED string.

In this manner you can adjust LED strings having mismatched numbers of LED by introducing a suitably compensating resistor in series with the respective LED strings.

Thus the problem is easily solved by changing the resistor value for the remaining smaller series.

This concludes our tutorial regarding how to connect LEDs in series and parallel for any given number of LEDs using a specified supply voltage, if you have any related query please use the comment box to get it solved.

Calculating LEDs in Series Parallel in Display Board

So far we leraned how LEDs may be connected or calculated in series and parallel.

In the following paragraphs we will investigate how to design a large numerical led display by joining LEDs in series and parallel.

As an example we will build a number display “8” using LEDs and see how it is wired.

Parts Required

You will need the following handful of electronic components for the construction:
RED LED 5mm. = 56 nos.
RESISTOR = 180 OHMS ¼ WATT CFR,
GENERAL PURPOSE BOARD = 6 BY 4 INCHES

How to Calculate and Construct LED Display?

The construction of this number display circuit is very simple and is done in the following way:

Insert all the LEDs in the general purpose board; follow the orientations as shown in the circuit diagram.

Initially solder only one lead of each LED.

After completing this, you will find that the LEDs are not aligned straight and are in fact fixed in quite a crooked manner.

Touch the soldering iron tip on the soldered LED point and simultaneously push the particular LED down so that its base is pushed flat on the board. Do this for all the LEDs to make them aligned straightly.

Now finish soldering the other unsoldered lead of each of the LEDs. Cut their leads cleanly with a nipper. According to the circuit diagram common up the positives of all the LED series.

Connect 180 Ohms resistors to the negative open ends of each series. Again, common up all the free ends of the resistors.

Mosfet In Series And Parallel

This ends the construction of the LED display number “8”. To test it, just connect a 12 volt supply to the common LED positive and the common resistor negative.

The number “8” should instantly light up in the form of a large numerical display and can be recognized even from long a distance.

Circuit Functioning Hints

To clearly understand how to design a large numerical led display it will be important to know the circuit functioning in details.

Looking at the circuit one may notice that the whole display has been divided into 7 LED series “bars”.

Each series contains a group of 4 LEDs. If we divide the input 12 volts with 4 we will find that each LED receives 3 volts enough to make them glow brightly.

The resistors make sure that the current to the LEDs is limited so that they may last long.

Now by just joining these series LEDs in parallel we can align them into different shapes to produce a huge variety of different alphanumeric displays.

Mosfet In Series And Parallel

Readers must have now easily understood how to calculate LED in different modes.

Connecting Mosfets In Parallel

Its just a matter of connecting LEDs first in series, then joining these in parallel connections and applying a voltage to their common positives and negatives.