PCB Routing guidelines - Part 2

Difference between Power Plane and Power Bus:

Layout engineers face different challenges in the real world. One of the biggest challenge is routing the power lines. A board with bigger stack up (meant more layers) will have a chance for separate power plane. Anyways with the present day board densities, and push for low cost designs, push for lesser number of layers is always on the cards. It is always a wise decision by the product management team to give the required freedom to the layout engineer as he is also a critical stakeholder for the product.

           Designs will always have more than one power rail to be routed on the board. PCB layout doesn't have enough space to have a single layer for each power rail. so, in the stack up power plane will be defined in one of the layer with a return path as ground in the next layer. The power plane will be split to accommodate the different power rails. It is the design strategy that need to be planned by the PCB layout engineer to have enough plane for each power rail to accommodate the current requirement. 

              Most of the times we have been talking about the space to route power rail on a separate plane. But this is not the case in smaller designs like for example, a 2-layer, 4-layer and sometimes 6-layer board. In this case, power rails can't have a separate plane but they have to routed as thicker traces to allow sufficient current to flow. 

The main disadvantage with power bus routing can be explained by a simple example:

Let us assume that a 5V rail is routed as power bus from the source to 3 chipsets. The following are the parameters defined in this scenario:

Voltage rail = 5V

Power Bus resistance = 200mohm (mainly because of trace width)

Each chip requires current = 1.5A, so, a total of 1.5*3 = 4.5A

So, 4.5A should flow through the power bus of 200mohm

Drop across power bus = 200mohm * 4.5 = 0.9V

That means, in power bus routing with above example, the 3rd device in the chain will only see 5-0.9 = 4.1V, which may not be sufficient excitation in most of the times.

As per the design rule, as trace width increases, impedance reduces. As impedance reduces, the drop reduces. This meant for power rails, we must have a wider plane to eliminate any drop due to trace routing. This is a clear indication that it is always safer to have power plane rather than having power bus and then distributing power to various devices on the board.

Understanding Boost Regulator

As the name suggests, Boost Regulator is a power supply component that boosts the input voltage to a desired value. Let us take a alkaline battery which gives a voltage of 1.5V.  In case, the circuit has to be operated at 3.3V, we have to use a boost converter in between to boost to a desired voltage of 3.3V. The basic boost converter circuit principle is based on the following circuit:

As shown in the above basic circuit of Boost converter, the main component in the Boost circuit are Inductor, Switch and a diode. The switch must be a low resistance semi-conductor device like a MOSFET.

Operation of the above circuit:

1. When the switch 'S' is closed, the current passes through the inductor through the switch 'S'. The inductor charges with + at the source side and - at the switch side. The inductor generates a magnetic field around it.
2. Once the switch is open, no current passes though switch 'S'. The charged inductor slowly discharges and the current now passes through the Diode 'D' to the load. The resultant current to the load increases as the source will be adding to the inductor current.
3. By the time inductor fully discharges, if the switch is closed, the inductor charges again. As the inductor is not allowed to discharge fully, the voltage across the load seen is sum of source voltage and inductor voltage. This meant that the output voltage is always greater than the source voltage.
4. The capacitor 'C' supplies to the load when the switch is closed. the diode in this case acts as blocking diode not allowing any current from capacitor pass through to the switch side.
5. So, when switch is closed, capacitor supplies to the load and when the switch is open, source+inductor supplies to the load and also charges the capacitor.
6. Input filter requirements are little relaxed as the supply sees a constant inductor load and there isn't discontinuous currents and need for filtering.

The diode used in this circuit is called "fly-back" diode. The fly-back diode is sometimes replaced by another low-resistance switch in low power applications. This is to avoid the diode losses.

The following parameters are to be checked while selecting a Boost converter:

1. Input voltage range
2. Output voltage (fixed/adjustable)
3. Output current
4. Switching frequency
5. Efficiency
6. Maximum switch current
7. Mosfet internal resistance
8. Quiescent current

All the above parameters are available from the datasheet of the boost converter ic.

Some of the important points of boost converter:

1. As the input voltage is reduced for a given fixed output voltage, the current drawn by the switch of the Boost converter increases, in this case the efficiency of the boost converter is less. The below figure from the data sheet of boost converter IC LM2621 illustrates the same:

2. At low load current conditions, the ripple current causes the inductor to discharge more quickly. This is the condition during which the modern step-up converters manage by varying the switching frequency.

Analog Circuits - PCB surface leakage current

The latest opamps in the market have good offset performance and very low bias currents. The bias current will be of the order of the 1pA and even of the order of fA. Take for example the latest precision opamp LMP7721 from Texas Instruments which has input bias current as low as 3fA. This is very low and requires very good layout design to take advantage of the good offset and drift performance.

                Leakage effect is one of the primary concerns in such circuits. The leakage can be through the PCB crosstalk which can easily draw more current than the input current drawn by the opamp pins. This is more prevalent in the high temperature and high voltage applications. Over a period of time the dust, humidity and other impurities formed over the PCB aggravates the problem and should be taken care with good layout practices. The impurities that we are talking here can be a flux residue which is left over due to improper PCB cleaning techniques. So, the following procedures are must in such critical applications:

1.       Surface coating on the PCB to avoid humidity, dust accumulation

2.       Using high quality dielectric materials

3.       Proper PCB cleaning techniques

4.       Good layout techniques

Example scenario:

Take for example a PCB trace whose impedance w.r.t given trace nearby is 110K and a 3.3V is applied across the main trace. In this case the current passing across that junction would be 30uA. If the input bias current of an opamp is comparable with this current, then definitely the circuit will misbehave. In such cases we have implement the following technique to avoid that leakage:

Have a guard trace around the trace carrying low current. This guard trace must have a potential very close to main trace. The below snapshot shows guard ring recommendation for the LMC6001 Ultra, Ultra-Low Input Current Amplifier.

Zigbee vs Zwave

PCB Fabrication and SMT Assembly - 2

For a beginner in electronics circuit design, there is always a bit of confusion regarding the manufacturing costs of PCB. We will always be looking for a common place where we can find the costs involving PCB fabrication and also factors to be considered while designing the same. So, once the layout engineer is ready with his Gerber, the designers will start looking for various fab options. They will be sending the Gerber to fab houses to get a quotation for the PCB fabrication. During the information exchange between companies and fab houses lot of technical information gets exchanged which finally decides the PCB fabrication cost. Following are the few points you have to look at before the PCB fabrication cost is decided.

1. PCB dimensions

2. Quanity required

3. Layer count of PCB

4. Fabrication material

5. PCB thickness

6. copper foil thickness on the PCB

7. Any special treatment required for the PCB

8. Trace width and line-to-line spacing

9. Via count and dimensions

10. Special PCB sizes

CNC machine

Are you a start up in the electronics industry? Tired of turn around times for the PCBs from the manufacturing houses? Thinking of doing a PCB prototypes by yourselves with a small setup? Thinking that fabricating a PCB using chemicals can be a messy job? Then CNC machine must be one of your choices.

               CNC short form for computer numerical control. These types of machines work automatically as per the instructions provided by the user. The manual techniques used in high production environments are long one and are replaced by CNC machines. Consider the PCB fabrication process where milling is done to create the signal routing patterns. PCB drilling, routing and engraving are done using CNC machines which can bring the prototypes out quickly and at a very low cost if done in-house. The CNC machine comprises of the mini computer or the microcomputer that acts as the controller unit of the machine. The existing CNC machines involve inputting a PCB Gerber where in the complete PCB process will be automatically done through a single machine. The entire process can be monitored over a well-defined GUI. The options when using a CNC machine is used involves checking of PCB material to be etched, drill bit to be used on the machine, the PCB tools (in fact output of PCB tools) supported by the CNC machine. the drill depth can be automated in the tool. The drill position is controlled through a well calibrated motors which meant no chance of any error. The RPM of the machine can be up to 110,000. Copper clad board is used instead of photo-resistant board. Here we are eliminating hazardous chemicals during the process. The process with CNC machine might be slow than normal etching but is always a quality and better environmental friendly option.

Few CNC machine vendors are:

1. Indus, INDIA

2.  Zen Tool works

3. Accurate CNC

4. Sahajanand

5. Advanced Technocracy Inc.

There are several manufacturers from China to check out.

High speed Designs - Part 3

Transmission lines is the first term when you hear while starting to work on high speed designs. Transmission line In PCB terminology, transmission line is a trace that connects various chips on the board. The transmission circuit is generally visualized as RLC circuit. The frequency response of any circuit depends on the R-L-C elements and they become predominant when used at high frequencies. 

Traditionally, engineers used interfaces like SPI, I2C, UART which are low speed interfaces. These interfaces didn't have issues with transmission line effects of the PCB traces. Unless the signals are routed over permitted lengths, there is no issue with maintaining the integrity of the signal. As signal frequencies increased, beyond 100MHz, with interfaces like Gigabit Ethernet, DDR, PCIe, etc the transmission line effects have to be considered. We generally read in design guidelines that the signal can't be routed for example, more than 1 inch and also the signal should have a recommended routing in the PCB, etc in the case of high speed signals. So, for a high speed signal even if you route shortest but don't take care of the recommended PCB routing guideline, you might end up with signal having signal integrity issues. Some of the major issues that we come across while working with high speed designs are:

Impedance mismatch

Reflections - overshoot, undershoot, ringing

cross-talk

Radiation

The basis of signal transmission comes from Maximum power transfer theorem of electrical circuit. This theorem states that the maximum power will be transferred from source to load when the source impedance matches the load impedance. If there is a mismatch, then power transferred to load gets reduced which meant there is more loss. In high speed design, when such scenario comes up, the source tries to send the signal to load and load will not be able to absorb the complete signal. Some part of the signal is reflected back to the source. This is what causes the reflections on the PCB. the signal when reflected back travels to source is reflected as there is a mismatch again. The signal hence forth travels between source and load while degrading over time. When these occur, the reflected signal adds/negates with the original signal causing the actual signal amplitude to increase/decrease. This is what we call undershoot/overshoot in high speed domain.