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Design Notes
*This design, and notes, borrow heavily from the wonderful words and wisdom presented by Paul Horowitz and Winfield Hill in The Art of Electronics (3 Ed.).1
The Art of Electronics (3ED) does not cover the calculation of components for the damping network ("Snubber") for R1, C1 as the subject will be covered in the upcoming "X Chapters" (publication date unknown). Quick research shows that this is a more complex task than one might expect. The authors give a range for components but it is unclear their power rating. It is indicated that the ring or spikes can be several tens of volts greater than the 36VRMS DC voltage. Most calculations for snubber network appear to consider the power stored in a inductor at the frequency of occurrence. The designed values of C and R are to achieve the desired dampening ratio. Authors of Art of Electronics indicated that there are many complexities interacting between the rectification, switching diodes, linear regulator and likely the transformer inductances. It is at this time beyond the activity of this design and "sane" values will be selected. Out of an abundance of safety, a metal film (not wire wound) 20 Ohm 3W resistor (digi PPC20W-3JCT-ND) was selected and a ceramic axial 0.1 uF capcitor rated for 100V (divi 399-9841-ND). Apparently different types of capacitors are preferred for this application but values large enough could not be easily located. Most straight forward reference found at http://www.cde.com/resources/catalogs/igbtAPPguide.pdf.
AC mains in the US for most residential households are 115Vrms, 60 Hz. The voltage can range from approximately 103.5Vrms on the low side to upwards of 130 Vrms on the high side. It is a good idea to at least consider the extremes when selecting components.
When choosing a transformer it is critical that the output voltage will never drop below the dropout voltage for the linear regulator. The dropout voltage is the amount by which the input voltage must exceed the regulated output voltage, for most L7800 regulators this is 2V. For the L7815 this is specified as V_d = 2V and for the L7915 V_d = 1.1 V.
When choosing a transformer try to keep the output DC voltage as low as possible to prevent power dissipation in the pass transistors of the linear regulators (remember power dissipation equals heat).
This design tries to keep the board layout and component selection general enough that it can be tailered to specific applications without requiring a new PCB to created.
In addition, the rectified and "smoothed" output will have some ripple, consider the lowest point in the ripple as input to the regulator. For this design, target 1V_pp ripple.
Transformers are typically specified for V_rms so the DC voltage will by \sqrt(2) greater, approximately 1.4 times. Then take into consideration the voltage drop in the bridge rectifier of two diodes. A typical diode has a 0.7V forward voltage drop. However, the GBU604 datasheet specifies V_fm=1.0 (Forward voltage drop per element).
Taking into account the desired output voltage from the regulator of 15V, the 2V dropout voltage of the regulator,the 1V peak-to-peak ripple of the filtered DC voltage, the 2V drop across the rectifier (2 diodes) comes out to a DC voltage of 20V. Add in additional 20-percent (24V) to account for transformer winding resistance and leakage inductance which reduce to loaded output voltage and a transformer that can source 17V_rms = 24V / \sqrt(2) is needed. When search for transformers, this will be indicated as a 34VCT (17V + 17V) center tapped transformer.
Choose a transformer with I_rms approximately twice the I_dc load. Current has not been discussed much. Linear regulators appear as constant current loads not restive. The L7815 and L1915 are rated for output currents up to 1.5A. So total output current of 3A between the two rails. Apply the twice current rule of thumb, a transformer that is rated for 6A is needed.
For example, a Stancor PN P-8674 chassis mount 36.0VCT, 6.0A, 117V may do the trick. At 8.3 pounds, it better!
Purchased Stancor P-8673 off ebay for $22.00. The P-8673 has a primary voltage of 117Vrms and secondary 36VCT, 4A. While slightly lower amperage than desired, it should supply what is required for this power supply's purpose. The target application requires the supply to source 0.5 A on the positive rail and approximately 50 mA on the negative rail. Design wise the only thing that really should change in the design is the smoothing capacitors. Recall
The rectified outputs of D1 (with respect to GND) contain significant ripple that must be smoothed out by large aluminum electrolytic capacitors before reaching the voltage regulator stage of the power supply. Capacitors should be chosen large enough to provide acceptably low ripple voltage but with high enough voltage rating sufficient to handle the worst case combination of no load on the transformer secondary and high line voltage (15%).
Under the following comments, the value of the capacitors (C) required for a specified ripple voltage (V) under full load current (I) can be conservatively approximated as C = I / (f * V). In short, the greater the load current the larger the capacitors need to be to maintain the specified ripple current.
- Ripple frequency of a full-wave rectified 60 Hz line main is 120 Hz (f) (60 Hz for half-wave).
- Ripple is small compared to output voltage of rectifier D1. Assume 1 Volt peak-to-peak is the specified ripple requirement.
- The downstream voltage regulators appear as constant-current load. For this design, 2 A load current selected even though larger than maximum current supply of the regulator.
- The discharge of the capacitor is modeled as a ramp.
C = (2 A) / (120 Hz * 1 Vpp)
C = ~16,667 uF
Tom's Hardware has a list of preferred capacitor vendors that includes Rubycon, Chemi-Con, Nichicon, Sanyo, Panasonic, Hitachi, Fujitsu and a few others. Tom's been around for a while and the brands suggested are well known.
Digikey 493-13922-ND Rubycon.
To protect equipment and anyone servicing the power supply, bleeder resistors are an important feature. In practice, a capacitor charges or decays to within 1% of its final value in five time constant (τ = RC). A good target is capacitor depletion in approximately 10 seconds.
The power dissipated in the voltage regulator is the difference in power times the current. The transformer outputs a voltage of 18*\sqrt(2) under load, less 2V drop from two diodes in the rectifier, less 1V drop from smoothing capacitors results in a voltage difference across the regulator of approximately 22.5V. Therefor the power dissipated at 1A is approximately 7.5 W. The TO-220 L7800 has a junction to case thermal resistance of 5 deg C/W. A heat sink rated at 2.6 deg C/W was selected as was a thermal pad with 0.5 deg C/W. Assuming an ambient temperature of 40 degrees C (104 deg F), the junction temperature is approximated as 100.75 deg C = 40 + (5 + 0.5 + 2.6)*7.5, which is well enough below the 125 max safe operating area of the regulator (125 deg C).
It is recommended that the regulator input be bypassed with capacitor if the regulator load capacitance is large. An input bypass capacitor should be selected to provide good high frequency characteristics to insure stable operation under all load conditions. A 0.33 uF or larger tantalum, mylar or other capacitor having low internal impedance at high frequencies should be chosen. The bypass capacitor should be mounted with the shortest possible leads directly across the regulator input terminals.2
No output bypass capacitor is needed for stability but it does improve transient response.1
In addition to protecting against reverse polarity connection caused by operator error or equipment failure, the diodes protect against latch-up problems.2
Great article on Matt's Tech Pages - Wire to Board Connector Info. Sums up a log of early questions regarding what connector series to invest in (both parts and tools). Matt compares the crimping results of the official MOLEX field tool and the $50 Japanese Engineer PA-21 available from many retailers including Amazon for crimping MOLEX Mini Fit Jr. connectors.
The MOLEX Mini Fit Jr. system features polarized (keying also available) connectors, positive locking latch (audible click), support up to 9A of current, 16-24 AWG, fully isolated plug and receptacle terminals, officially support male and female terminals, multiple points of contact, single and double row plugs and receptacles share crimp terminals with standard Mini-Fit and Mini-Fit Plus, and provide products intended for wire-to-wire and wire-to-board configurations.
Note that at time of writing the KiCAD footprint library Connectors_Molex.pretty only contained dual row footprints.